GitLab

The workflow definitions are much nicer than before, thanks to an existing GitHub Action for setting up Conda (actually mambaforge) it's mostly the same for all three platforms. 
Plus @pymor/all can finally restart those pipelines on failure. 
Test jobs now take around 30% less time compared to the azure setup. Mainly due to environment caching. No more weird exit code plugin hacks either.

Fix Weekly CI jobs

instead of having the more opaque, manual patching in some modules
this also hides the monkeying from the pytest output