Hints/stubs should be explicit about returning None

With disallow_untyped_calls = True set for mypy I get various reports about untyped methods. For example, QWidget.show().

qtrio/examples/emissions.py:98: error: Call to untyped function "show" in typed context

Which is stubbed as follows.

def show(self): ...

It would be nice if these were explicitly hinted as returning None.

def show(self) -> None: ...