Add initial OpenAPI 3.1 support

[stephenfin]

This isn't my finest work but it passes with the schema I'm using locally. There are quite a few changes in OpenAPI 3.1 but two are addressed here:

• 'nullable' is gone in favour of the 'null' type and type arrays
• 'paths' is now optional and spec files can contain only components

There's a whole lot more to support here including things like webhooks but we focus on the MVP here. All that extra functionality can be added later.

A list of commits:

• Add initial framework for OpenAPI 3.1
• OAS 3.1: Support specs without 'paths'
• OAS 3.1: Support null type
• OAS 3.1: Prefer non-null values
• tox: Allow running specific tests
• Update OpenAPI-Specification submodule
• Add tests for OpenAPI 3.1 schemas