Support 2023-07 config

[gkreitz]

This PR implements reading 2023-07-draft problem.yaml files.

The main implementation is in the new problemtools.metadata module, where I created pydantic models and dataclasses to represent the config file format. For each supported version (2023-07 and legacy), there is a model which should precisely allow what the standard allows. We then convert to a model called Metadata, which is a unified format, to allow code later to access metadata in a unified way, regardless of problem format version. The unified format is just a stricter version of 2023-07 with some legacy fields added.

To get type safety in verifyproblem, I added problem.getMetadata() to read the metadata, as opposed to re-using the get() function, which passes dicts.

I think we should probably take another look at how Problem is initialized and how ProblemAspects communicate information between each other. For instance, I would like to make it a fatal error when we fail to parse config.yaml (there really is not much point in proceeding), but I couldn't come up with a clean way to do that in the current structure (and I don't want to mix that refactor into this already large PR).

This PR bumps the python version we require to 3.11, since I wanted to use StrEnum to represent e.g. our various license options.