www/py-django-tables2: Update to 2.7.5

• Support for Python 3.8 was dropped with the 2.7.1 release, adjust USES=python accordingly.

• Switch to the PEP517 build framework.

Changelog since 2.7.0:

https://github.com/jieter/django-tables2/blob/v2.7.5/CHANGELOG.md