Use gitlab-ci to build API documentation and run tests

This patch series

• builds API docs based on docstrings (tidying only a couple of doc strings that were generating errors; there is lots more that could be done to improve the docstrings and other documentation could be added too)
• enables the building of the API docs automatically, hosting them on pages.debian.net
• enables running the test suite
• moves the version string inside the module so that it is available to the documentation

Comments welcome!