There is no swh directory in swh-environment repository so
the mypy check was failing when committing changes to a
Python test file.

this is needed as individual packages might need specific plugins
(e.g., for very dynamic libraries like django, sqlalchemy, etc.) and
there is no guarantee those plugins will always be compatible

plus, per-package configuration is what we are already doing for other
test/lint tools, e.g., tox, pytest, etc. Diverging for mypy does not
seem a great idea