fix: add support for Django 4.0

[browniebroke]

• Add Django 4.0 to the test matrix
• Update readme to mention support for 4.x
• Update Trove classifiers

[browniebroke]

Looks like the tests are failing for Django tip (upcoming 4.1). While it's nice to know about these, it seems a bit out of scope of what I'm trying to achieve here... Is there a way to mark these failures as "acceptable" and not fail the build?

[nedbat]

I don't have a way to exclude them yet, and am looking for someone to help fix them instead. But I can see it's fine to merge this PR. Thanks.

[browniebroke]

Regarding the failures with 4.1, I noticed that the tests fail when running the whole suite, but they seem to pass when each test is executed separately (pytest tests/test_extends.py::BlockTest::test_inheriting_with_unused_blocks).