Mypy: Set disallow_untyped_defs = true

[fohrloop]

Set disallow_untyped_defs = true. This is a bit stricter policy than before.

• Disallows defining functions without type annotations or with incomplete type annotations
• Also disallows mismatched function signatures; if a method is defined def foo(self, a=1),
  if is not possible to use it in subclass as super().foo(*args, **kwargs). This discourages
  the usage of *args, **kwargs in places where they're not really needed.

See: https://mypy.readthedocs.io/en/stable/config_file.html#confval-disallow_untyped_defs