Add static field to base, update tests to run on 3.11, fix 3.11 compatibility

[daniel-dodd]

This PR comprises the following changes:

• (1) Add static_field to base, and use this in the testing.

• (2) Limit simple_pytree imports to 3 places (and the tests only):

  • (a)gpjax.base
  • (b)gpjax.linops.linear_operator (We should update this to a module in future).
  • (c)gpjax.dataset (We should update this to a module in future).

There should be no other imports.

• Will need to remove simple_pytree imports from the docs_update branch in a separate PR.
• Perhaps ought to consider (in a future PR), only using Module and limiting simple_pytree import to gpjax.base.

This PR also fixes compatibility issues with Python 3.11, that requires dataclass fields to be hashable (while jax.Arrays are not...).

[frazane]

Looks good to me!