Build and test infrastructure update

This MR does a few things all at the same time

• rewrite setup.py and setup.cfg and provide pyproject.toml to define the metadata and build steps
• abandon setuptools_scm in favour of a hardcoded version number (for now)
• add a /pyRing/tests/ directory with a basic pytest harness and a few functional tests
• add a gitlab-ci configuration to run the tests and a simple example

Note: this is a duplicate of !1 (closed) which done broke.