site-packages in .venv directory aren't ignored by linter

There's a default ignore pattern (`**/site-packages/**/*.py`) that's supposed to take of this, but it doesn't match files/dirs that start with a dot. This affects a lot of python projects (including this one -- `Contributing.md` recommends using that name), since `.venv` is a standard name for the virtualenv dir.

There's a simple fix (#3948)