Skip test_readonly_save on Cygwin.

The test seems to require opening a file for reading, mmapping it,
then opening that file for writing.  Windows doesn't allow this.

Author: DWesl

Tests/test_image.py

@@ -193,6 +193,10 @@ def test_internals(self):
         assert not im.readonly
 
     @pytest.mark.skipif(is_win32(), reason="Test requires opening tempfile twice")
+    @pytest.mark.skipif(
+        sys.platform == "cygwin",
+        reason="Test requires opening an mmaped file for writing",
+    )
     def test_readonly_save(self, tmp_path):
         temp_file = str(tmp_path / "temp.bmp")
         shutil.copy("Tests/images/rgb32bf-rgba.bmp", temp_file)