Add type hints for Image.open, Image.init, and Image.Image.save

[nulano]

Fixes #7941

While the Image.open function is currently marked as returning an Image object, it does so by calling a factory function from OPEN, which is marked as returning an ImageFile:

Pillow/src/PIL/Image.py

Lines 222 to 228 in e8ab564

	OPEN: dict[
	str,
	tuple[
	Callable[[IO[bytes], str | bytes], ImageFile.ImageFile],
	Callable[[bytes], bool] | None,
	],
	] = {}

The accept function usually returns a bool, but I found one case (webp) where it can return a str instead to be used as a warning.

[aclark4life]

Hmmm, jinx #7943 ?