Tidy .gitlab-ci.yml, fix py3.5 test, add py2.7 test (allowed to fail)

• Update .gitlab-ci.yml to make it tidier and easier to add the package build phase.
• Change test names to show more useful info in the amount of text normally shown in the job name (Python interpreter version first, abbreviated Ubuntu version second).
• Moved the different Python interpreter versions from the before_script into the sections that specifically require them, to minimise dependencies for each test.
• Update tox.ini so that missing interpreters are not skipped (py3.5 was being skipped and the test was coming back as a pass -- people can always skip this in their command lines or we can add to ./run-tests if people want it), but this seems the more sensible behaviour for CI.
• Add Python 3.5 test based on Xenial (Ubuntu 16.04) because Python 3.5 is not available in the Bionic repos.
• Add a Python 2.7 test back in (with an allow_failure:true flag so that it does not stop our tests passing while we know it is not working)