www/py-django-treebeard: Update to 4.5.1

• Also adjust/remove relevant variables to Mk/Uses/pytest.mk which was recently added.

Changelog since 4.4:

https://github.com/django-treebeard/django-treebeard/blob/4.5.1/CHANGES.md

MFH: No (feature release, pytest.mk not present in 2022Q2)