Add deprecations helper

[hugovk]

I noticed in a project using Pillow that the deprecation warning resulted in a very long traceback:

=================================== FAILURES ===================================
  ___________________________________ test_pil ___________________________________
  
      def test_pil():
          image_manager = PILImageManager("RGB")
          osm = OSMManager(image_manager=image_manager)
          image, bounds = osm.create_osm_image((30, 31, -117, -116), 9)
          wh_ratio = float(image.size[0]) / image.size[1]
  >       image2 = image.resize((int([80](https://github.com/hugovk/osmviz/runs/5822426177?check_suite_focus=true#step:5:80)0 * wh_ratio), 800), Image.ANTIALIAS)
  
  test/functional/test_manager.py:11: 
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  
  name = 'ANTIALIAS'
  
      def __getattr__(name):
          deprecated = "deprecated and will be removed in Pillow 10 (2023-07-01). "
          categories = {"NORMAL": 0, "SEQUENCE": 1, "CONTAINER": 2}
          if name in categories:
              warnings.warn(
                  "Image categories are " + deprecated + "Use is_animated instead.",
                  DeprecationWarning,
                  stacklevel=2,
              )
              return categories[name]
          elif name in ("NEAREST", "NONE"):
              warnings.warn(
                  name
                  + " is "
                  + deprecated
                  + "Use Resampling.NEAREST or Dither.NONE instead.",
                  DeprecationWarning,
                  stacklevel=2,
              )
              return 0
          old_resampling = {
              "LINEAR": "BILINEAR",
              "CUBIC": "BICUBIC",
              "ANTIALIAS": "LANCZOS",
          }
          if name in old_resampling:
  >           warnings.warn(
                  name
                  + " is "
                  + deprecated
                  + "Use Resampling."
                  + old_resampling[name]
                  + " instead.",
                  DeprecationWarning,
                  stacklevel=2,
              )
  E           DeprecationWarning: ANTIALIAS is deprecated and will be removed in Pillow 10 (2023-07-01). Use Resampling.LANCZOS instead.
  
  .tox/py310/lib/python3.10/site-packages/PIL/Image.py:[82](https://github.com/hugovk/osmviz/runs/5822426177?check_suite_focus=true#step:5:82): DeprecationWarning
  ----------------------------- Captured stdout call -----------------------------

https://github.com/hugovk/osmviz/runs/5822426177?check_suite_focus=true

I was going to shorten them using f-strings, but thought it could be useful to create a deprecation helper function.

[nulano]

Seems reasonable. Maybe add this to https://github.com/python-pillow/Pillow/blob/main/docs/reference/internal_modules.rst?

[hugovk]

Good idea! I tidied up the params for RTD.

https://pillow--6184.org.readthedocs.build/en/6184/reference/internal_modules.html#module-PIL._deprecate