diff --git a/.github/workflows/test-docker.yml b/.github/workflows/test-docker.yml
index 3e8a416a5..2b32bcfce 100644
--- a/.github/workflows/test-docker.yml
+++ b/.github/workflows/test-docker.yml
@@ -17,6 +17,7 @@ jobs:
           debian-10-buster-x86,
           centos-6-amd64,
           centos-7-amd64,
+          centos-8-amd64,
           amazon-1-amd64,
           amazon-2-amd64,
           fedora-30-amd64,
diff --git a/.travis.yml b/.travis.yml
index 9dea5fb14..936e64520 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -40,6 +40,7 @@ matrix:
     - env: DOCKER="debian-10-buster-x86" DOCKER_TAG="master"
     - env: DOCKER="centos-6-amd64" DOCKER_TAG="master"
     - env: DOCKER="centos-7-amd64" DOCKER_TAG="master"
+    - env: DOCKER="centos-8-amd64" DOCKER_TAG="master"
     - env: DOCKER="amazon-1-amd64" DOCKER_TAG="master"
     - env: DOCKER="amazon-2-amd64" DOCKER_TAG="master"
     - env: DOCKER="fedora-30-amd64" DOCKER_TAG="master"
diff --git a/CHANGES.rst b/CHANGES.rst
index 2e0f10845..8693df466 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,6 +5,12 @@ Changelog (Pillow)
 7.0.0 (unreleased)
 ------------------
 
+- Raise ValueError for io.StringIO in Image.open #4302
+  [radarhere, hugovk]
+
+- Fix thumbnail geometry when DCT scaling is used #4231
+  [homm, radarhere]
+
 - Use default DPI when exif provides invalid x_resolution #4147
   [beipang2, radarhere]
 
diff --git a/Tests/test_image.py b/Tests/test_image.py
index 83da76b96..47e7420ef 100644
--- a/Tests/test_image.py
+++ b/Tests/test_image.py
@@ -1,3 +1,4 @@
+import io
 import os
 import shutil
 import tempfile
@@ -91,6 +92,9 @@ class TestImage(PillowTestCase):
     def test_bad_mode(self):
         self.assertRaises(ValueError, Image.open, "filename", "bad mode")
 
+    def test_stringio(self):
+        self.assertRaises(ValueError, Image.open, io.StringIO())
+
     def test_pathlib(self):
         from PIL.Image import Path
 
diff --git a/Tests/test_image_reduce.py b/Tests/test_image_reduce.py
index 5ed6ccaa3..d8f2ce1ec 100644
--- a/Tests/test_image_reduce.py
+++ b/Tests/test_image_reduce.py
@@ -94,7 +94,7 @@ class TestImageReduce(PillowTestCase):
                 # rotate previous image
                 band = bands[-1].transpose(Image.ROTATE_90)
                 bands.append(band)
-            # Correct alpha channel to exclude completely transparent pixels.
+            # Correct alpha channel by transforming completely transparent pixels.
             # Low alpha values also emphasize error after alpha multiplication.
             if mode.endswith("A"):
                 bands[-1] = bands[-1].point(lambda x: int(85 + x / 1.5))
@@ -102,7 +102,7 @@ class TestImageReduce(PillowTestCase):
         else:
             assert len(mode_info.bands) == 1
             im = self.gradients_image.convert(mode)
-        # change the height to make a not square image
+        # change the height to make a not-square image
         return im.crop((0, 0, im.width, im.height - 5))
 
     def compare_reduce_with_box(self, im, factor):
@@ -196,7 +196,7 @@ class TestImageReduce(PillowTestCase):
         for factor in self.remarkable_factors:
             self.compare_reduce_with_reference(im, factor, 0.8, 5)
 
-        # With opaque alpha, error should be way smaller
+        # With opaque alpha, an error should be way smaller.
         im.putalpha(Image.new("L", im.size, 255))
         for factor in self.remarkable_factors:
             self.compare_reduce_with_reference(im, factor)
@@ -219,7 +219,7 @@ class TestImageReduce(PillowTestCase):
         for factor in self.remarkable_factors:
             self.compare_reduce_with_reference(im, factor, 0.8, 5)
 
-        # With opaque alpha, error should be way smaller
+        # With opaque alpha, an error should be way smaller.
         im.putalpha(Image.new("L", im.size, 255))
         for factor in self.remarkable_factors:
             self.compare_reduce_with_reference(im, factor)
diff --git a/Tests/test_lib_pack.py b/Tests/test_lib_pack.py
index 91479e343..f9e6e7c4c 100644
--- a/Tests/test_lib_pack.py
+++ b/Tests/test_lib_pack.py
@@ -377,7 +377,7 @@ class TestLibUnpack(PillowTestCase):
         self.assert_unpack(
             "RGBA",
             "RGBa;16L",
-            b"\x88\x01\x88\x02\x88\x03\x88\x00" b"\x88\x10\x88\x20\x88\x30\x88\xff",
+            b"\x88\x01\x88\x02\x88\x03\x88\x00\x88\x10\x88\x20\x88\x30\x88\xff",
             (0, 0, 0, 0),
             (16, 32, 48, 255),
         )
@@ -392,7 +392,7 @@ class TestLibUnpack(PillowTestCase):
         self.assert_unpack(
             "RGBA",
             "RGBa;16B",
-            b"\x01\x88\x02\x88\x03\x88\x00\x88" b"\x10\x88\x20\x88\x30\x88\xff\x88",
+            b"\x01\x88\x02\x88\x03\x88\x00\x88\x10\x88\x20\x88\x30\x88\xff\x88",
             (0, 0, 0, 0),
             (16, 32, 48, 255),
         )
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index dd77f3278..6ddbb08ff 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -51,6 +51,11 @@ jobs:
     docker: 'centos-7-amd64'
     name:   'centos_7_amd64'
 
+- template: .azure-pipelines/jobs/test-docker.yml
+  parameters:
+    docker: 'centos-8-amd64'
+    name:   'centos_8_amd64'
+
 - template: .azure-pipelines/jobs/test-docker.yml
   parameters:
     docker: 'amazon-1-amd64'
diff --git a/docs/installation.rst b/docs/installation.rst
index 929bc951a..57c4f8344 100644
--- a/docs/installation.rst
+++ b/docs/installation.rst
@@ -400,6 +400,8 @@ These platforms are built and tested for every change.
 +----------------------------------+--------------------------+-----------------------+
 | CentOS 7                         | 3.6                      |x86-64                 |
 +----------------------------------+--------------------------+-----------------------+
+| CentOS 8                         | 3.6                      |x86-64                 |
++----------------------------------+--------------------------+-----------------------+
 | Debian 9 Stretch                 | 3.5                      |x86                    |
 +----------------------------------+--------------------------+-----------------------+
 | Debian 10 Buster                 | 3.7                      |x86                    |
diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index 3417aeb4a..866c89831 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -1797,9 +1797,9 @@ class Image:
            If the image has mode "1" or "P", it is
            always set to :py:attr:`PIL.Image.NEAREST`.
            See: :ref:`concept-filters`.
-        :param box: An optional 4-tuple of floats giving the region
-           of the source image which should be scaled.
-           The values should be within (0, 0, width, height) rectangle.
+        :param box: An optional 4-tuple of floats providing
+           the source image region to be scaled.
+           The values must be within (0, 0, width, height) rectangle.
            If omitted or None, the entire source is used.
         :param reducing_gap: Apply optimization by resizing the image
            in two steps. First, reducing the image in integer times
@@ -1874,15 +1874,15 @@ class Image:
 
     def reduce(self, factor, box=None):
         """
-        Returns reduced in `factor` times copy of the image.
+        Returns a copy of the image reduced by `factor` times.
         If the size of the image is not dividable by the `factor`,
         the resulting size will be rounded up.
 
         :param factor: A greater than 0 integer or tuple of two integers
-            for width and height separately.
-        :param box: An optional 4-tuple of ints giving the region
-           of the source image which should be reduced.
-           The values should be within (0, 0, width, height) rectangle.
+           for width and height separately.
+        :param box: An optional 4-tuple of ints providing
+           the source image region to be reduced.
+           The values must be within (0, 0, width, height) rectangle.
            If omitted or None, the entire source is used.
         """
         if not isinstance(factor, (list, tuple)):
@@ -2783,12 +2783,20 @@ def open(fp, mode="r"):
        and be opened in binary mode.
     :param mode: The mode.  If given, this argument must be "r".
     :returns: An :py:class:`~PIL.Image.Image` object.
-    :exception IOError: If the file cannot be found, or the image cannot be
-       opened and identified.
+    :exception FileNotFoundError: If the file cannot be found.
+    :exception PIL.UnidentifiedImageError: If the image cannot be opened and
+       identified.
+    :exception ValueError: If the ``mode`` is not "r", or if a ``StringIO``
+       instance is used for ``fp``.
     """
 
     if mode != "r":
         raise ValueError("bad mode %r" % mode)
+    elif isinstance(fp, io.StringIO):
+        raise ValueError(
+            "StringIO cannot be used to open an image. "
+            "Binary data must be used instead."
+        )
 
     exclusive_fp = False
     filename = ""
diff --git a/src/PIL/PngImagePlugin.py b/src/PIL/PngImagePlugin.py
index 9f98cd3d7..ee1400d67 100644
--- a/src/PIL/PngImagePlugin.py
+++ b/src/PIL/PngImagePlugin.py
@@ -882,7 +882,6 @@ def _save(im, fp, filename, chunk=putchunk):
             b"\x01",
         )
 
-    info = im.encoderinfo.get("pnginfo")
     if info:
         chunks = [b"bKGD", b"hIST"]
         for cid, data in info.chunks:
diff --git a/winbuild/build_dep.py b/winbuild/build_dep.py
index 5762de5e5..778570139 100644
--- a/winbuild/build_dep.py
+++ b/winbuild/build_dep.py
@@ -225,8 +225,8 @@ set DefaultPlatformToolset=v100
     if bit == 64:
         script += (
             r"copy /Y /B "
-            + r'"C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Lib\x64\*.Lib" '
-            + r"%%FREETYPE%%\builds\windows\vc2010"
+            r'"C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Lib\x64\*.Lib" '
+            r"%%FREETYPE%%\builds\windows\vc2010"
         )
         properties += r" /p:_IsNativeEnvironment=false"
     script += (