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

@@ -13,6 +13,9 @@ ignore =
 
 
 [mypy]
+show_error_codes = true
+enable_error_code = ignore-without-code
+
 disallow_subclassing_any = True
 
 # Avoid creating future gotchas emerging from bad typing