core: improve mypy coverage

my/core/warnings.py

@@ -6,12 +6,13 @@ E.g. would be nice to propagate the warnings in the UI (it's even a subclass of
 '''
 
 import sys
+from typing import Optional
 import warnings
 
 # just bring in the scope of this module for convenience
 from warnings import warn
 
-def _colorize(x: str, color=None) -> str:
+def _colorize(x: str, color: Optional[str]=None) -> str:
     if color is None:
         return x
 
@@ -28,8 +29,7 @@ def _colorize(x: str, color=None) -> str:
         # todo log something?
         return x
 
-
-def _warn(message: str, *args, color=None, **kwargs) -> None:
+def _warn(message: str, *args, color: Optional[str]=None, **kwargs) -> None:
     stacklevel = kwargs.get('stacklevel', 1)
     kwargs['stacklevel'] = stacklevel + 2 # +1 for this function, +1 for medium/high wrapper
     warnings.warn(_colorize(message, color=color), *args, **kwargs)