Use latest python image for the gitlab CI.

Changed the default CI python image to latest, which currently translates to Python 3.9.7.

Added optional pipelines for running tests of multiple older python versions (so if you want to test CLEASE on python 3.6, the pipeline may be triggered manually). I didn't enable these to run by default, as that would take up too many CI resources. Optionally, we could probably figure out a way to always trigger them when targeting the default branch (i.e. master), but I think that's unnecessary.

Default CI now runs on the latest python version (except the GUI, which can only handle up to python 3.7).

I triggered Python 3.6-3.8 manually for demonstration purposes in this MR.