build: add '.git' to 'make lint-py' exclude list

When run locally [flake8](http://flake8.pycqa.org) was creating false positives by scanning the __.git__ directory.  This PR prevents that behavior.

PR-URL: https://github.com/nodejs/node/pull/24802


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>