www/py-django-bootstrap3: Update to 15.0.0

• Add "do-test" target to make future QA easier.

Changelog:

• Drop support for Django 3.0
• Add support for Django 3.2.
• Fix render_alert
• Add docs and tests to sdist

MFH: 2021Q3
(cherry picked from commit dbe3ced23da5b10a56866bf0d0798e09a8e6988e)