Use the `jsonschema==4.0` interface and test against new schema versions

[Zac-HD]

See https://github.com/Julian/jsonschema/blob/main/CHANGELOG.rst#v400 and update tests etc. to check for the (partially) supported Drafts 2020-12 and 2019-09. Also maybe the False != 0 validation fix will allow some test skips or workarounds to be removed?

array items have changed semantics in new drafts too, yay; we'll need to test against more schemas and update gen_schemas to support the new keywords 😭

[thatguysimon]

Hi @Zac-HD!
What's the latest on this? I'd be happy to help 👍🏻

[Zac-HD]

It's waiting on volunteers, so your contribution would be very appreciated! I'd suggest a multi-step process, so that PRs can stay small and reviewable:

• get currently-supported schema versions working with jsonschema >= 4. At this point we can bump the minimum version, which should make things easier
• add new schemas to the test suite, and then make sure that the new keywords are supported. This is probably the fiddlier part - fortunately the tests are really sensitive, so while you might need to extend them for new features I'm confident that if they pass, the code is fine.