Python: Automatically set virtualenv paths

When using `mypy` as linter, it needs to know where to look for packages. This breaks when using virtual environments.

Manually starting vim with:

MYPYPATH=/path/to/venv/lib/pythonX.Y/site-packages vim

will make it work. But this is tedious and I often forget it.

It would be nice if ALE was able to detect the path and automatically set it before running m`mypy`.