Remove krr's dependency on matplotlib, and pil (pillow)

[aantn]

Today we have a transitive dependency on matplotlib and pil, which causes all sorts of problems. For example:

• krr 1.17.0 install and upgrade with brew on MacOS reports libpng linkage errors #358
• KRR doesn't support newer python version (i.e. python3.13) #435
• Fedora brew install causes undefined symbol OPENSSL 3 #360

We want to remove that dependency. I looked through our poetry.lock file and it seems like the dependency on matplotlib and pil come only from prometheus-api-client. (matplotlib/pil appear in other 'extra' dependencies, which is fine and doesn't force a dependency for us AFAIK)

There are upstream issues for this in prometheus-api-client:

• Ways to import PrometheusConnect without also importing **huge** pandas and matplotlib 4n4nd/prometheus-api-client-python#225
• Resolved issue #225 4n4nd/prometheus-api-client-python#293

But they haven't been fixed for years, so I suggest forking prometheus-api-client and fixing it in our fork so we can have an immediate solution.