Make Image.__array__ take optional dtype argument

This is required by the numpy protocol.

Tests/test_image_array.py

@@ -14,6 +14,10 @@ def test_toarray():
         ai = numpy.array(im.convert(mode))
         return ai.shape, ai.dtype.str, ai.nbytes
 
+    def test_with_dtype(dtype):
+        ai = numpy.array(im, dtype=dtype)
+        assert ai.dtype == dtype
+
     # assert test("1") == ((100, 128), '|b1', 1600))
     assert test("L") == ((100, 128), "|u1", 12800)
 
@@ -27,6 +31,9 @@ def test_toarray():
     assert test("RGBA") == ((100, 128, 4), "|u1", 51200)
     assert test("RGBX") == ((100, 128, 4), "|u1", 51200)
 
+    test_with_dtype(numpy.float)
+    test_with_dtype(numpy.uint8)
+
     with Image.open("Tests/images/truncated_jpeg.jpg") as im_truncated:
         with pytest.raises(OSError):
             numpy.array(im_truncated)

src/PIL/Image.py

@@ -681,7 +681,7 @@ class Image:
             raise ValueError("Could not save to PNG for display") from e
         return b.getvalue()
 
-    def __array__(self):
+    def __array__(self, dtype=None):
         # numpy array interface support
         import numpy as np
 
@@ -700,7 +700,10 @@ class Image:
         class ArrayData:
             __array_interface__ = new
 
-        return np.array(ArrayData())
+        arr = np.array(ArrayData())
+        if dtype is not None:
+            arr = arr.astype(dtype)
+        return arr
 
     def __getstate__(self):
         return [self.info, self.mode, self.size, self.getpalette(), self.tobytes()]