Make GitLab CI mirror GitHub CI

The motivation

As a GitLab user, I would like my CI to be as sophisticated as the CI I get if I use GitHub. At the moment, our test PyPI install script is very different in GitHub vs. GitLab. It would be great to have these two have feature parity.

The proposed solution

• Get GitLab and GitHub up to feature parity

Alternatives

Leave things as is, and just live with the difference.

Additional context

There are some things we won't manage in any case (e.g. running stuff on Windows via GitLab) so we won't reach total feature parity.