Update comments for operator.itemgetter.__call__ generic following mypy 1.11 fix

[Avasam]

python/mypy#14032 has been fixed in mypy 1.11.0 (I didn't go as far as to search for the exact commit).

This updates operator.itemgetter to include a change I originally wanted to do in #9117, removes the outdated comment, adds a comment explaining why we still can't do that change, and adds a regression test.

[Avasam]

There seems to be a variance issue in the interaction with map and/or groupby. But I honestly can't quite figure it out.

[srittau]

I'm also not sure why this doesn't work. Still I think for now it's best to postpone (i.e. close) this PR, until this works better.

[Avasam]

If you close this, please make it an issue. The behaviour here feels unexpected and seems worth investigating.

[Avasam]

I don't mind abandoning the changes of this PR if I can at least update the reason why we have to keep def __call__(self, obj: SupportsGetItem[Any, Any]) -> Any: ..., given that python/mypy#14032 is closed as completed.

I just don't fully understand what's the issue at hand.

[srittau]

This could be a variance issue. SupportsGetItem's second type var is covariant, while _T is invariant. Maybe introduce a _T1_co type var and try to use that instead?

[Avasam]

I took another look at it, and I think I understand my main issue: There's no way to preserve the itemgetter typevar as a literal
itemgetter(0) should be a itemgetter[Literal[0]] but type-checkers understand it as a is a itemgetter[int].

groupby iterates tuples (tuple[_T_co, Iterator[_S_co]]). But there's no way afaik to use map(operator.itemgetter(0), groupby(iterable)) such that type checkers know that it returns a map of the first element (map[_T_co] and not map[_T_co | Iterator[_S_co]])

(I think getter: operator.itemgetter[Literal[0]] = operator.itemgetter(0) technically works, but it's even more verbose and you still deal with variance issues with groupby anyway)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉