Annotate cat_list()

[glatterf42]

This PR comes from me wanting to showcase a small, completely typed function in the UBA presentation on Tuesday as an example for how to use docstrings. I didn't want to spend too much time looking, cat_list() seems fine, but is missing some annotations. This PR provides them.

How to review

No need to merge this (quickly), I'm mainly interested in the docs built from this PR. But of course, this is a small improvement we can bring in anytime :)

• Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• [ ] Add or expand tests; coverage checks both ✅ Just adding annotations.
• Add, expand, or update documentation.
• [ ] Update release notes. Just adding annotations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.6%. Comparing base (5c0b44f) to head (8d3b772).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #896   +/-   ##
=====================================
  Coverage   95.6%   95.6%           
=====================================
  Files         46      46           
  Lines       4337    4337           
=====================================
  Hits        4147    4147           
  Misses       190     190           

Files with missing lines	Coverage Δ
message_ix/core.py	97.7% <100.0%> (ø)