Be stricter about mypy needing error codes

These make it clearer what's being ignored and harder to
accidentally ignore more than expected.

setup.cfg

@@ -31,6 +31,9 @@ max-line-length = 100
 # Exclude our copies of external stubs
 exclude = ^jedi/third_party
 
+show_error_codes = true
+enable_error_code = ignore-without-code
+
 # Ensure generics are explicit about what they are (e.g: `List[str]` rather than
 # just `List`)
 disallow_any_generics = True