Migrate from Travis to GitHub Actions and add upper bounds in requirements.txt

[gdubicki]

with testing, publishing to PyPI and creating a release in GitHub.

Note that notifications to IRC have NOT been migrated yet. Let's do that in a separate PR. (And maybe someone who is using IRC unlike me would be better to do it...?)

Notes:

• To make publishing to PyPI work please set PYPI_API_TOKEN secret in the GitHub repo config,
• Uncomment line 29 in .github/workflows/publish-to-pypi.yml to test releasing to PyPI (note: I have not tested if this actually works - in my project I just made a few pre-releases).

[gdubicki]

You can see how the GitHub Actions outputs will look like here: https://github.com/gdubicki/puppetboard/actions