Add a mandatory settings for setting "--python-version" on mypy

[sametmax]

Expected Behaviour

Mypy checks the proper types for my code

Actual Behaviour

Mypy checks the types for the version of Python it's installed in against my code, resulting in weird errors.

Steps to Reproduce

Install mypy with "python3.5 -m pip install mypy", setup anaconda for using mypy withtout any particular settings, and see it fails like in this github issue: https://github.com/python/mypy/issues/4222#issuecomment-342916300

ST3, Anaconda and OS versions

ST3143, latest version of anaconda as of this date, Ubutun 16.04.

ST3 Console Logs

No errors.

Anaconda's JsonServer Logs

No errors in Logs.

[DamnWidget]

All mypy support needs to be updated.