PIL buffer test: relax copy assert to warning

Our test suite should not fail if PIL improves in this regard,
esp. seeing as test suite failure inhibites new releases.

tests_old/test_renderer.py

@@ -3,6 +3,7 @@
 
 import math
 import numpy
+import warnings
 import PIL.Image
 import pytest
 import pypdfium2 as pdfium
@@ -385,6 +386,7 @@ def test_pil_nocopy_where_possible(bitmap_format, rev_byteorder, is_referenced,
 
     assert pil_image.getpixel((0, 0)) == pixel_a
     bitmap.buffer[0] = val_b
+
     if is_referenced:
 
         # changes to the buffer are reflected in the image
@@ -396,4 +398,7 @@ def test_pil_nocopy_where_possible(bitmap_format, rev_byteorder, is_referenced,
         assert bitmap.buffer[0] == val_a
 
     else:
-        assert pil_image.getpixel((0, 0)) == pixel_a
+        if pil_image.getpixel((0, 0)) == pixel_b:
+            warnings.warn(f"PIL now references {bitmap.mode} mode bitmaps.")
+        else:
+            assert pil_image.getpixel((0, 0)) == pixel_a