Remove pydantic v1 support

[lilyminium]

Does anyone need Pydantic < 1 support? @fjclark or @JMorado ? It'd be great to drop this in the tests.

[fjclark]

No, I don't need Pydantic 1 support. Thanks for checking.

[JMorado]

I don't need it either -- thanks!

[lilyminium]

Thanks both!