Improvements to GitHub Actions workflows

[leouieda]

PR #194 introduces GitHub Actions as our continuous integration for most systems, replacing Azure Pipelines. We're still using TravisCI to do the deployment to PyPI and Github Pages. A few things that could be improved:

• Deploy packages from the master branch to test.pypi.org. This let's us check the PyPI upload and catch problems before we try to do a release.
• Deploy to PyPI on tag builds.
• Deploy the documentation pages to gh-pages on master and tag builds. This will likely require an adaptation of our current scripts in continuous-integration. We might be able to repurpuse an existing Action instead.
• Add a job to test on Python 3.9-dev. Would need to use the system Python instead of miniconda and install packages with pip.
• Use reviewdog to make comments inline from flake8 output (see this comment)

The PyPI deployments can be done with pypi-publish (see this comment for details).

Of all of these, the first 3 are a priority since they would allow us to move away from TravisCI for good.

[hugovk]

1. Deploy packages from the master branch to test.pypi.org. This let's us check the PyPI upload and catch problems before we try to do a release.
2. Deploy to PyPI on tag builds.

Here's an example:

• https://github.com/hugovk/tinytext/blob/master/.github/workflows/deploy.yml

[leouieda]

Deployment to PyPI was implemented in #196 and to GitHub Pages in #198.

[leouieda]

Closing this since we don't really need the reviewdog bit.