Ensure Image and core palettes are in sync after PA conversion (alphas dropped)

Co-authored-by: Andrew Murray <radarhere@users.noreply.github.com>

Author: akx

Tests/test_image_convert.py

@@ -359,7 +359,6 @@ def test_matrix_identity() -> None:
     assert_image_equal(converted_im, im)
 
 
-@pytest.mark.xfail(reason="zombie colors from palette shouldn't reappear")
 def test_pa2p_truly_drops_alpha() -> None:
     im = Image.frombytes("P", (2, 1), bytes([0, 1])).convert("PA")
     im.putpalette(bytes([255, 0, 0, 7, 0, 255, 0, 9]), "RGBA")

src/PIL/Image.py

@@ -1242,9 +1242,8 @@ def convert_transparency(
 
         new_im = self._new(im)
         if mode in ("P", "PA") and palette != Palette.ADAPTIVE:
-            from . import ImagePalette
-
-            new_im.palette = ImagePalette.ImagePalette("RGB", im.getpalette("RGB"))
+            # Install the original image's palette into the new copy
+            new_im.putpalette(im.getpalette("RGB"))
         if delete_trns:
             # crash fail if we leave a bytes transparency in an rgb/l mode.
             del new_im.info["transparency"]