Callables whose return value is ignored

def foo(callback):
    callback()

There's a few ways to type this function:

def foo(callback: Callable[[], None]) -> None: ...
def foo(callback: Callable[[], Any]) -> None: ...
def foo(callback: Callable[[], object]) -> None: ...

Callable[[], None] is not great, because it doesn't allow you to pass a callback that does return something; that will also work at runtime, and the return value is ignored. When I learned about this, I changed my callbacks to use Callable[[], Any] instead. But Callable[[], object] would be even better.

We could check for this, but does it belong to this tool? This lint isn't really stub-specific.

No source file, test, or entry point is named. First resolve whether checking ignored callable return values belongs in this stub-focused plugin, then define the intended treatment of Callable[[], None], Callable[[], Any], and Callable[[], object] before implementation and tests.