Ignore mypy configuration file

[paw-lu]

Nox does not have a mypy.ini or setup.cfg. This will cause mypy to look for configuration files in other directories, which can cause failures when testing locally. This PR adds the --config-file= argument, which tells mypy to ignore config files.

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Thanks!

[cjolowicz]

Thanks for catching this @paw-lu! This LGTM, I'm just wondering if we shouldn't move the mypy command-line options from the Noxfile to our own mypy.ini.

[theacodes]

Thanks for catching this @paw-lu! This LGTM, I'm just wondering if we shouldn't move the mypy command-line options from the Noxfile to our own mypy.ini.

Maybe, but I think for now we're fine. It'd be a nice follow-up if someone wants to tackle that.