Support PEP 612: Parameter Specification Variables

[rchen152]

PEP: https://www.python.org/dev/peps/pep-0612/

As a simple first step, we should add ParamSpec and Concatenate to the typing stub so typeshed can start using them (python/typeshed#4827).

[rchen152]

Assigning to myself to implement the "first step" described above, to unblock typeshed.

[rchen152]

It's now possible to use a ParamSpec or Concatenate as the first argument to a Callable in type stubs.

[cebtenzzre]

If the test_paramspec_args testcase is expanded slightly to actually use the arguments, pytype-single prints ERROR:pytype.matcher Invalid type: <class 'pytype.abstract.function.ParamSpecMatch'> and then raises NotImplementedError: ParamSpecArgs. This is with pytype 2024.10.11. Is this the expected behavior?

from typing import Callable, ParamSpec, TypeVar
P = ParamSpec("P")
T = TypeVar("T")

def f(x: Callable[P, T], /, *args: P.args, **kwargs: P.kwargs) -> T:
    return x(*args, **kwargs)

def foo():
    pass

f(foo)