Add caching for python dependencies in ci.yml

[henninggaertner]

• fixes (in part) the runtime of the ci on github, saving some 15 odd seconds of installing libraries using pip

• this particular change adds a step called "cache python dependencies", which on the first run (as seen here) doesn't find the cached dependencies, but on a re-run uses the previously downloaded deps

• the cache is updated, whenever the (hash of) requirements.txt changes, which is quite rare.

PR checklist

• main-branch has been merged into local branch to resolve conflicts
• tests and linter have passed AFTER local merge
• at least one other dev reviewed and approved
• documentation
• tests

[github-actions]

Coverage report

The coverage rate went from 70.21% to 69.55% ⬇️
The branch rate is 63%.

None of the new lines are part of the tested code. Therefore, there is no coverage data about them.

[antonneubauer]

Looks good to me. The benefits seem pretty small but maybe that'll change with future dependencies