Check return types

[radarhere]

This is part of #8362 - I'm hoping to break down that PR into easier-to-review chunks.

1. Assert getpixel() returns a tuple, before using index access, since it might also return a float or None.
   

   Pillow/src/PIL/Image.py

   Lines 1645 to 1647 in a370209

   	def getpixel(
   	self, xy: tuple[int, int] | list[int]
   	) -> float | tuple[int, ...] | None:

2. Assert that getcolors() does not return None, because checking length or using index access.
   

   Pillow/src/PIL/Image.py

   Lines 1415 to 1417 in a370209

   	def getcolors(
   	self, maxcolors: int = 256
   	) -> list[tuple[int, tuple[int, ...]]] | list[tuple[int, float]] | None:

3. Assert that getpalette() does not return None before slicing a list.
   

   Pillow/src/PIL/Image.py

   Line 1577 in a370209

   def getpalette(self, rawmode: str | None = "RGB") -> list[int] | None: