tkinter.font.Font: make __getitem__ and cget behave the same

[Akuli]

#4891 replaced widget.cget(Literal) with widget.cget(str), making cget behave just like __getitem__. This is the same change for Font objects, except that if a known Literal is passed, then the return type is more specific than Any and that may help catch bugs.

Examples:

• font.cget('size') has type int (not changed)
• font['size'] has type int (previously Any)
• After setting foo: str = 'size', font.cget(foo) has type Any (previously error)
• After setting foo: str = 'size', font[foo] has type Any (not changed)

[Akuli]

Stubtest complains about how def __getitem__(self, option) and def __getitem__(self, key) differ. I think type checkers ignore that for __dunder__ methods (at least mypy), so I whitelisted it. If that's not acceptable, another alternative is to replace __getitem__ = cget with copy/pasta.

[srittau]

In reality, dunder methods are position-only. We should probably change our stubs accordingly at some point.