Merge CI fixes from master

This MR synchronizes .gitlab-ci.yml with master and merges minor changes, that fix non-essential errors (failing PyFlakes check and obsolete link in documentation).

Instead of cherry-picking all CI-related commit to master, all changes are grabbed as a single commit, because cherry-picking is going to take too much time (there are too many commits, and merge conflicts are going to appear) without bringing any practical benefit.