FIX: account for constant deprecations in Pillow 9.1

https://pillow.readthedocs.io/en/stable/deprecations.html#constants
https://pillow.readthedocs.io/en/stable/releasenotes/9.1.0.html#constants

Image.None -> Image.Dither.None
Image.ADAPTIVE -> Image.Palette.ADAPTIVE

Author: tacaswell

lib/matplotlib/backends/backend_pdf.py

@@ -1700,8 +1700,19 @@ 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 = (
+                    Image.Dither.NONE if hasattr(Image, 'Dither')
+                    else Image.NONE
+                )
+                pmode = (
+                    Image.Palette.ADAPTIVE if hasattr(Image, 'Palette')
+                    else 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")