Add a GitHub Action to run tox

[cclauss]

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

============================= test session starts ==============================
platform linux -- Python 3.12.1, pytest-7.4.4, pluggy-1.3.0
cachedir: .tox/py/.pytest_cache
rootdir: /home/runner/work/radon/radon
plugins: mock-3.12.0
collected 404 items

radon/tests/test_cli.py ..............                                   [  3%]
radon/tests/test_cli_colors.py ...                                       [  4%]
radon/tests/test_cli_harvest.py ....................                     [  9%]
radon/tests/test_cli_tools.py ............                               [ 12%]
radon/tests/test_complexity_utils.py ................................... [ 20%]
........................................................................ [ 38%]
....                                                                     [ 39%]
radon/tests/test_complexity_visitor.py ................................. [ 47%]
................................                                         [ 55%]
radon/tests/test_halstead.py ...........                                 [ 58%]
radon/tests/test_ipynb.py .....                                          [ 59%]
radon/tests/test_other_metrics.py ...................................... [ 69%]
........................................................................ [ 86%]
..                                                                       [ 87%]
radon/tests/test_raw.py ................................................ [ 99%]
...                                                                      [100%]

============================= 404 passed in 0.88s ==============================

[akx]

(See also #258 😄)