Release 0.3.1

[marcofavorito]

Release summary

Version number: 0.3.1

Release details

• Improved CI: using GitHub Actions instead of Travis.
• Included many other checks in CI: safety, black, liccheck.
• Improved documentation.
• Made the project PEP 518 compliant

Checklist

Put an x in the boxes that apply.

https://github.com/whitemech/pythomata/blob/master/CONTRIBUTING.md

• I have read the CONTRIBUTING.md file
• I am making a pull request against the master branch (left side), from release-<version-number>
• I've updated the dependencies versions in Pipfile to the latest, wherever is possible.
• Lint and unit tests pass locally
• I built the documentation and updated it with the latest changes
• I've added an item in HISTORY.md for this release
• I bumped the version number in the __version__.py file.
• I published the latest version on TestPyPI and checked that the following command work:
  pip install pythomata==<version-number> --index-url https://test.pypi.org/simple --force --no-cache-dir --no-deps
• After merging the PR, I'll publish the build also on PyPI. Then, I'll make sure the following
  command will work:
  pip install pythomata==<version_number> --force --no-cache-dir --no-deps
• I tagged the merge commit with v<version-number>
• I published the docs: mkdocs gh-deploy

Further comments