Ignore mypy complain about incompatible override

We override the app property to get the correct
typing for our "Clive" application class.

Error:
- clive/__private/ui/widgets/clive_widget.py:23: error: Signature of "app" incompatible with supertype "MessagePump"  [override]
  clive/__private/ui/widgets/clive_widget.py:23: note:      Superclass:
  clive/__private/ui/widgets/clive_widget.py:23: note:          App[object]
  clive/__private/ui/widgets/clive_widget.py:23: note:      Subclass:
  clive/__private/ui/widgets/clive_widget.py:23: note:          Clive