It turns out we can actually use pydantic with a minimum version of

2.10, since Python 3.8 support wasn't removed until 2.11. Given that we

want to be as forward-looking as possible, this change sets the minimum

version to that. On 3.9 and newer we'll actually end up pulling in the

latest.
This change also fixes several type annotation issues which were

breaking when running against Python 3.8. Most of these were inside the

Pydantic model definitions for GitLab CI, but there was one cast()

call.

Ran unit tests on all supported versions of Python.