[build] modify GitHub Actions config to run tox and publish to PyPI

[sloria]

see https://github.com/4Catalyzer/flask-resty/blob/master/.github/workflows/main.yml

[webknjaz]

That example has some problems (security / outdated software / long-living API token). Here's my PyPUG guide that should be easier to follow: https://packaging.python.org/en/latest/guides/publishing-package-distribution-releases-using-github-actions-ci-cd-workflows/

[sloria]

closed in #130