feat: Warn about unused mypy configuration

See
<https://mypy.readthedocs.io/en/latest/command_line.html#cmdoption-mypy-warn-unused-configs>.

Author: l0b0

mypy.ini

@@ -11,4 +11,5 @@ show_error_codes = True
 strict_equality = True
 warn_redundant_casts = True
 warn_return_any = True
+warn_unused_configs = True
 warn_unused_ignores = True