migrate to gh actions, add tests and publish wheel

[thatlittleboy]

Fixes #43. Tested on my own fork with test-PyPI repository.

You can see the successful GH actions here:

• pytests: https://github.com/thatlittleboy/mkdocs-markdownextradata-plugin/actions/runs/4016216932
• publish to pypi: https://github.com/thatlittleboy/mkdocs-markdownextradata-plugin/actions/runs/4016266690

Wheel can be successfully installed (using py 3.10 locally)

---

Changes

• Drop support for python 3.5 and below. The plugin code already uses f-strings, so the code won't run properly on lower versions anyway. https://github.com/rosscdh/mkdocs-markdownextradata-plugin/blob/24492bc6df7dbc58b17ab7aabb80acf0d10e7e5d/markdownextradata/plugin.py#L56-L58

• I made the executive decision to move from Travis CI to GH actions, no particular reason other than I've no experience with Travis.

  • Tests will run on all supported python versions (3.6 till 3.11); On every commit to a pull request and on commits to the master branch.
  • Release to PyPI will be on Release creation. That's just the general practice I see, feel free to edit the trigger to suit your own workflow.

• I bumped the version number in setup.py to 0.25.0 since it seems you already made a release on GH with that tag but pypi is still 0.24.0.

IMPT: You'll need to set up your PYPI_USER and PYPI_PASSWORD repository secrets so the CI can pick up on those variables before twine upload.

[rosscdh]

Fantastic contribution! Thank you 🙇