devel/py-python-gitlab: update to 6.1.0

While here, use USE_PYTHON=pytest for tests instead of overriding
do-test target.

Changelog: https://github.com/python-gitlab/python-gitlab/releases/tag/v6.1.0

PR: 287864
Reported by: Derek Schrock <dereks@lifeofadishwasher.com> (maintainer)