mypy_config option should be relative to project root

[mwillsey]

The mypy_config option should be relative to the project root. For example, I should be able to do the following in my setup.cfg:

[mypy]
...
[flake8]
mypy_config=setup.cfg

That should point mypy to this same setup.cfg. But if you run flake8 from a folder inside the project, setup.cfg gets literally passed in as the config file to mypy, and it can't find it.

This line seems to be the problem. This should be making the path relative to the project root.