Resolve "Create a release using CI/CD"

Closes #17 (closed)

List of changes:

• .gitlab-ci.yml: Before_script is only executed for the tests/linters jobs. They are in the same stage. A new stage (release) is added that is triggered by tagging. The Build job will deploy the package to the Package Registry. The Release job releases the code.
• pyproject.toml: A toml file is added to automatically build the package and write the __version__.py, based on git tagging (using the setuptools_scm module)
• setup.py: the version is removed here
• __init__.py: the version is retrieved here, from the automatically generated __version__.py

\approve @ds @gislars