CI: Add a cygwin run to GitHub Actions.

Requested for the _imagingtk fix.

.github/workflows/test-cygwin.yml

@@ -0,0 +1,48 @@
+name: Test Cygwin
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: windows-2019
+    name: Cygwin Python 3.8
+
+    steps:
+      - name: Checkout Pillow
+        uses: actions/checkout@v2
+      - name: Set up Cygwin
+        uses: egor-tensin/setup-cygwin@v3
+        with:
+          platform: x64
+          packages: >
+            python38-devel python38-tkinter libfreetype-devel
+            libimagequant-devel libjpeg-devel liblcms2-devel
+            libopenjp2-devel libraqm-devel libtiff-devel libwebp-devel
+            libxcb-devel python38-olefile python38-pathlib
+            python38-numpy netpbm ImageMagick jpeg xorg-server-extra
+            xorg-server-common xinit python38-setuptools
+            python38-wheel
+          install-dir: 'C:\tools\cygwin'
+      - name: Clean up path
+        uses: egor-tensin/cleanup-path@v2
+        with:
+          dirs: 'C:\tools\cygwin\bin;C:\tools\cygwin\lib\lapack'
+      - name: Build Pillow
+        shell: 'C:\tools\cygwin\bin\dash.exe'
+        run: |
+          /usr/bin/python3.8 setup.py bdist_wheel
+      - name: Install Pillow
+        shell: 'C:\tools\cygwin\bin\dash.exe'
+        run: |
+          /usr/bin/python3.8 -m pip install Pillow-*-cp38-cp38-cygwin_*_x86_64.whl
+      - name: Test Pillow
+        shell: 'C:\tools\cygwin\bin\dash.exe'
+        run: |
+          /usr/bin/python3.8 selftest.py
+          xvfb-run -s '-screen 0 1024x768x24' .ci/test.sh