Proposal: absorb `django-filter-stubs` into typeshed

[intgr]

The django-filter-stubs project has has signalled that they want to be merged into typeshed, project author @DavisRayM agreeing with it: DavisRayM/django-filter-stubs#13

I'm interested in implementing this.

I propose we do this in a way that preserves original commits and authorship information. I can do some git magic to "transplant" upstream commits to this repo, in the correct directory structure, but skipping changes/commits that don't touch .pyi files. And then you can merge that without squashing.

But if you want to squash merge and aren't interested in preserving history, then let me know and I won't bother with that.

[AlexWaygood]

Preserving history sounds like a good idea to me, if it's possible.

Merge commits aren't enabled for this repository (the only two enabled ways of merging are "squash and merge" and "rebase and merge"), but I think repo admins might be able to override that? (I'm not a repo admin, so can't say for sure.)

[srittau]

We can enable merge commits when needed. (Sidenote: I just deleted a webhook that was still pointing to travis-ci, but not surprisingly failing.)