Replace test and doc requirements.txt files with optional dependencies in pyproject.toml

[edwardchalstrey1]

I realised that a better way than having these two separate sets of requirements in their own files at doc/requirements.txt and tests/requirements.txt we can add them as optional dependencies for pygambit in pyproject.toml, something like:

[project.optional-dependencies]
test = ["pytest"...]
doc = ["ipython"...]

TODO:

• Update pyproject.toml and remove requirement files
• Include ruamel.yaml (see Create initial games catalog from contrib/games #731)
• Update contributor docs page with how to install with doc or test dependencies (and catalog docs if required)
• Update relevant GitHub actions