Added ImageType as alias for Image.Image

[radarhere]

Helps #2625.

Resolves #6676. Also suggested in #7635 (review)

When using the Image class as a type hint, you must currently use from PIL import Image; Image.Image.

This PR allows for from PIL import ImageType; ImageType.

[nulano]

I wasn't sure how this would affect the docs, but it looks good:

Should ImageType be documented somewhere? Not sure how it would be done, since I couldn't find a type alias directive in sphinx: https://www.sphinx-doc.org/en/master/usage/domains/python.html

[radarhere]

Ok, I've added it to the docs - https://pillow--7639.org.readthedocs.build/en/7639/reference/Image.html

See below.

[hugovk]

I know I suggested this in the first place, but after some thought, perhaps it's not so useful after all.

Image.Image vs ImageType is adding abstraction on top of something which isn't particularly complex in the first place, only save two chars, at the cost of another layer of indirection.

I can see aliases may be more useful for more complex things, such as tuples of ints for RGB etc, but I think we should pass on this one.