Add support settings from setup.cfg

[pbelskiy]

Hello, thanks for your package, but could you please add support to read settings from mypy related section from setup.cfg because many python machinery tools do so, and I don't like to have a lot of different config files in root of my repo. ✌️

[dmtucker]

That should be supported (i.e. you should be able to use any configuration file supported by Mypy) 🤔

Is something not working as expected?

[pbelskiy]

How can I pass config file path? mypy has --config-file but, how to use it throught pytest-mypy plugin?