server: upgrade to python 3.12

[austin3dickey]

We currently run CPython 3.11 in the server. 3.12 recently came out and has interesting support for more natural-feeling type declarations. It might be interesting to modernize https://github.com/conbench/conbench/blob/main/conbench/types.py, for example, or this code:

conbench/conbench/entities/_entity.py

Lines 67 to 70 in 2dc20b9

	T = TypeVar("T", bound="EntityMixin")
	class EntityMixin(Generic[T]):