www/py-django-auth-ldap: Update to 5.0.0

• Support for Python 3.8 was dropped with release 5.0.0, adjust USES=python accordingly as Python 3.8 will remain in the Ports tree until the end of October.

Changelog:

https://github.com/django-auth-ldap/django-auth-ldap/releases/tag/5.0.0

MFH: No (contains breaking changes)