CI: Always install django-stubs from git master

[intgr]

It's not uncommon to need to coordinate changes between django-stubs and djangorestframework-stubs projects. E.g. I need to make some changes in django-stubs first and then also make a similar change here that builds on django-stubs. Happens particularly often with mypy updates like #416.

So I propose making django-stubs permanently a git dependency in CI and development requirements.txt

[intgr]

A bit offtopic but related: Most of the problems with using older django-stubs are failures in our CI that do not occur in actual user projects (in my experience).

But there's a risk of creating a dependency on other changes already merged in django-stubs and we don't test against multiple django-stubs versions. So I'm thinking going forward we should consider bumping DRF-stubs dependency on django-stubs with every DRF-stubs release.

[sobolevn]

Yes, seems reasonable. We already synchronize releases, so it can decrease the amount of problems during development with no real risks.