GitHub Action to run tox

[cclauss]

Test results: https://github.com/cclauss/six/actions

Fixes: #345

• switch to GH actions #345

Run tox on Python 2.7 and 3.5 to 3.13-dev and PyPy 2.7 and 3.10 building from the excellent GitHub Actions work done for https://github.com/dateutil/dateutil

      matrix:
        os: [ubuntu-latest, windows-latest, macos-latest]
        python-version: [
          "2.7",
          "3.5",
          "3.6",
          "3.7",
          "3.8",
          "3.9",
          "3.10",
          "3.11",
          "3.12",
          "3.13-dev",
          "pypy-2.7",
          "pypy-3.10",
        ]

34 jobs completed successfully in 3m 23s.

@benjaminp @hroncok Your reviews, please? It would be good to get automated tests back in place on all the Python versions and platforms. It would also be useful to get Windows contributors to focus on the failing test_getoutput().