Update version of pygambit installed by GH actions and add to dev dependencies

[edwardchalstrey1]

In .github/workflows/test.yml it has:

    - name: Install dependencies
      run: |
        python -m pip install --upgrade pip
        pip install -e .[dev]
        pip install git+https://github.com/gambitproject/gambit.git # TODO: add pygambit to dev dependencies after 16.5 release.

Update this after pygambit 16.5 release to pull from pip by updating the optional dependencies in pyproject.toml to include pygambit:

[project.optional-dependencies]
dev = ["pytest>=7.0.0", "pytest-cov", "nbformat", "nbclient", "ipykernel"]