build: Show mypy error codes

This helps in determining the error codes to ignore if necessary
without having to re-run the build.

tox.ini

@@ -4,7 +4,7 @@ skipsdist = True
 [testenv]
 commands =
   python3 -m yapf --diff --recursive phile setup.py test_phile
-  python3 -m mypy .
+  python3 -m mypy --show-error-codes .
   python3 -m coverage erase
   python3 -m coverage run
   python3 -m coverage report