fix build issues/warnings spotted in last release

[zsoldosp]

https://github.com/zsoldosp/django-currentuser/actions/runs/21146848776/job/61000515234?pr=87 but probably present before that (build for #87)

• Django system check warnings
• tox env install warnings (ignore shallow git in ci, setuptools vs pyproject versions)
• make test lint docs missing python3.13 warning (RELEASE_PYTHON in Makefile)