Handle mypy error codes

[cdleonard]

Recent versions of mypy added string error codes next to the messages. I don't remember seeing this in older versions.

If you have older pytest-mypy-testing assertions you have two options:

• Adjust all assertion messages
• Use "hide_error_codes = True" to make them still pass.

It would be nice if this plugin would parse the message from mypy and allow matching based on error codes. Stability is not guaranteed but they're still likely to be more stable that messages. The fact that error codes are short strings would also alleviate #29.