www/py-django-bitfield: Update to 2.1.0

Changelog:

• Add support for Django 3.1, 3.2 (No changes needed).
• Add support for Python 3.8, 3.9.
• Fixed multiple bugs with use in the Django admin.
• Removed dead compatibility code.

PR: 261317
Approved by: rm (maintainer)
MFH: 2022Q1

(cherry picked from commit 1f812b94c835bfd7550110a8865b61efb051fe25)