Automate Release process with Github Actions

[Flexonze]

For reference, see this comment.

Right now, the release process is manual and looks like this:

# Make sure to update the version number in setup.py

# Add the new changes to CHANGELOG.md

# build the package
python -m build

# Push it to pypi
python -m twine upload --skip-existing dist/*

# Enter pypi token

It would be nice to automate some of this process with Github Actions, as long as we keep it simple and easy.