Add flit build and artifact steps

Adds a build step to the ci which runs flit build and uploads the resulting files as an artifact.

I decided to rename the built files, removing version information from them, as the info is redundant (stored in the artifact url) and makes it harder to download from the url just with a version number/branch/tag.

The default (and I assume our?) GitLab behaviour is that artifacts expire after some default time (90 days? It should say in the web interface), unless they are associated with a tag or are the latest artifact for a branch. Because of that I didn't restrict this stage to only run on a tag or release, but that can be added in with only: tags.