[Proposal] Scheduled daily github workflow to run tests using Django's main branch

[sarahboyce]

Was listening to DjangoChat and how we don't get enough testers for pre-releases. It was also mentioned that folks may not know how to do this

I think djangoproject.com should lead by example. We should have our tests run against Django main everyday with warnings enabled. Other folks can then look at this workflow as an example for their own project. We are also contributing to testing pre-released Django 👍