Add ReturnType-like utility for getting function return types

This is a feature request, so if there is a way to achieve this already, please point me and future me in the right way.

In TypeScript, we can use the ReturnType utility type to get the return type of a function to reuse in other places. This is very useful to make the code short, readable, and easy to change if necessary, especially in cases where I want to wrap the call of a function from another module in some other function.

An example of how it might be used:

# module: foo.py
def foo() -> None | baz.Baz | tuple[int, int]:
    ...

# module: bar.py
def bar() -> ReturnType[foo.foo]:
    ...

In this case, I could maybe give the return type a name like FooReturn and import that in bar, but that seems less elegant rather than just reusing the function I'm already using.

Note that if automatic or manual type inference (https://github.com/python/mypy/issues/4409) was implemented, most of my usages would be covered.

まず、この issue で要求されている ReturnType に似たユーティリティと、自動または手動の型推論に関するリンク先の mypy issue #4409 を確認します。foo.py と bar.py の例に対して意図された型構文と動作を判断します。done は、要求された戻り値型の再利用がサポートされているか、既存のサポート対象のアプローチが文書化されていることを意味するものとします。