feat: Warn about unused mypy configuration

l0b0 · l0b0 · commit 74542550b0f7 · 2021-07-08T10:40:47.000+12:00
See <https://mypy.readthedocs.io/en/latest/command_line.html#cmdoption-mypy-warn-unused-configs>.
diff --git a/mypy.ini b/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
