Support mypy 1.12

[glatterf42]

Our CI failed today because of the release of mypy 1.12.
This PR collects changes to adjust. It also includes a suggested change, namely to activate local_partial_type = true since this is planned to become the default with mypy 2.0. However, this setting does not seem to affect this repository too much :)

This PR is likely a prerequisite for other PRs as they won't be able to clear the code quality check before being rebased on this one.

How to review

• 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 adjusting to dependency.
• [ ] Add, expand, or update documentation. Just adjusting to dependency.
• [ ] Update release notes. Just adjusting to dependency.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.5%. Comparing base (7700a82) to head (fd9530b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #882   +/-   ##
=====================================
  Coverage   95.5%   95.5%           
=====================================
  Files         46      46           
  Lines       4340    4340           
=====================================
  Hits        4148    4148           
  Misses       192     192           

Files with missing lines	Coverage Δ
message_ix/tools/add_year/__init__.py	66.7% <ø> (ø)

[khaeru]

I added one commit so that the mypy version in .pre-commit-config.yaml is up to date with the one we use/comply with.

Beyond that, good to go.