Automatically publish version tags to PyPI via GH Actions

[CasperWA]

This is blocked until #92 has been merged and needs to be rebased. Currently, only the last four commits are related to this PR.

Introduce a new workflow for publishing on PyPI via GitHub Actions.
The workflow steps:

• Run for pushes on tags (no matter if on master branch or not).
  • Run only if repository is 'Materials-Consortia/optimade-python-tools'
  • Run for tags of 'v[0-9]+.[0-9]+.[0-9]+*
    The last * makes it possible to tag alpha, beta, and release candidate versions. Basically it means the workflow will run with anything after the initial part of the tag name has been verified.
• Checkout repository, install Python 3.7, and update setuptools and install optimade with base dependencies (pip install -e .).
• Assert package version equals tag name, i.e., "v" + __version__ matches the pushed tag name.
  • If this does not fail, print a helpful message. Most importantly one MUST remove the newly created tag from GitHub and the local repository.
• Build source distribution.
• Upload/publish to TestPyPI.
  • This is to check that uploading to PyPI works and that the version does not already exist on PyPI, since this will make the upload fail.
• Finally, upload/publish to PyPI.

[ml-evs]

Cheers for this @CasperWA! Hard to test, but everything here looks sensible and self-verifying. Will leave it to you to merge in whatever order you want.