Merge pull request matplotlib#22780 from meeseeksmachine/auto-backport-of-pr-22766-on-v3.5.x

Backport PR matplotlib#22766 on branch v3.5.x (FIX: account for constant deprecations in Pillow 9.1)

Author: oscargus

azure-pipelines.yml

@@ -134,6 +134,9 @@ stages:
       - script: env
         displayName: 'print env'
 
+      - script: pip list
+        displayName: 'print pip'
+
       - bash: |
           PYTHONFAULTHANDLER=1 python -m pytest --junitxml=junit/test-results.xml -raR --maxfail=50 --timeout=300 --durations=25 --cov-report= --cov=lib -n 2 ||
             [[ "$PYTHON_VERSION" = 'Pre' ]]

lib/matplotlib/backends/backend_pdf.py

@@ -1718,8 +1718,13 @@ def _writeImg(self, data, id, smask=None):
                 # Convert to indexed color if there are 256 colors or fewer
                 # This can significantly reduce the file size
                 num_colors = len(img_colors)
-                img = img.convert(mode='P', dither=Image.NONE,
-                                  palette=Image.ADAPTIVE, colors=num_colors)
+                # These constants were converted to IntEnums and deprecated in
+                # Pillow 9.2
+                dither = getattr(Image, 'Dither', Image).NONE
+                pmode = getattr(Image, 'Palette', Image).ADAPTIVE
+                img = img.convert(
+                    mode='P', dither=dither, palette=pmode, colors=num_colors
+                )
                 png_data, bit_depth, palette = self._writePng(img)
                 if bit_depth is None or palette is None:
                     raise RuntimeError("invalid PNG header")