Open API 3.1.0 support

[jefflembeck]

All Submissions:

• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Have you written unit tests?
• [x] Have you written unit tests that cover the negative cases (i.e.: if bad data is submitted, does the library respond properly)?
• [x] This PR is associated with an existing issue?

Closing issues

This addresses number #794.

Put closes #XXXX (where XXXX is the issue number) in your comment to auto-close the issue that your PR fixes.

Potential Problems With The Approach

There is probably way too much here. I cargo culted a ton of things over from 3.0.0. I also made some changes to how certain types extend others.

Test plan

Broke out tests and fixtures strictly for 3.1. Happy to make changes to that, but wanted something we could start talking out earlier rather than later.

[jefflembeck]

My real issue I wanted to get to the bottom of was getting variadic tuples in. But the differences in prefixItems and items and what is possible for 3.1.0 vs 3.0.0 made me shoot for the moon a little.

[bioc-druzgami]

Any news?

[WoH]

I started on this, but I am very much beyond my capacity until October unfortunately. This one's a big improvement, and I need want to make time to fix the CI, dependencies and rebase this one onto a green CI.