Adopt `ParamSpec` for type hints

[squahtx]

PEP-612 adds ParamSpec, which will allow us to correctly annotate:

• Decorators which preserve parameters and return types. These will no longer require casts.
• Decorators which change the return type can be correctly annotated and type checked.
• Basically anything that handles *args and **kwargs.

mypy 0.931
(blocked on #11712)

As of mypy 0.931, basic ParamSpec usage is supported, plus P.args and P.kwargs. These styles of function can be annotated:

def decorate(f: Callable[P, R]) -> Callable[P, Optional[R]]:
    def wrapped(*args: P.args, **kwargs: P.kwargs) -> Optional[R]:
         ...

def call(f: Callable[P, R], *args: P.args, **kwargs: P.kwargs) -> R:
    ...

Concatenate and def call(f: Callable[P, R], *args: P.args, some_flag: bool = False, **kwargs: P.kwargs) (extra kwarg) don't work yet.

[squahtx]

mypy 0.950 has been released, and includes support for Concatenate. See #12585.

[DMRobertson]

Probably the best way to action this is to grep the source tree for Callable\[\.\.\. or Callable[^\[]]. i.e. looking for Callable[..., <stuff>] and Callable without any square brackets.