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.

Inizia esaminando l'utility simile a ReturnType richiesta in questa issue e l'issue #4409 di mypy collegata sull'inferenza automatica o manuale dei tipi. Determina la sintassi dei tipi e il comportamento previsti per l'esempio con foo.py e bar.py; done deve significare che il riutilizzo richiesto del tipo di ritorno è supportato oppure che l'approccio già supportato è documentato.