Update Python support for 3.13

[glatterf42]

As suggested by @khaeru in #880, we should drop support for the now outdated version 3.8 of Python and add support for the new 3.13. This PR does just that.

Please also see the analogous iiasa/ixmp#544, which details some issues that need to be closed upstream before we can support 3.13.

How to review

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

PR checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.6%. Comparing base (d3cf5bd) to head (10b9e8d).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #881   +/-   ##
=====================================
  Coverage   95.5%   95.6%           
=====================================
  Files         46      46           
  Lines       4340    4327   -13     
=====================================
- Hits        4148    4139    -9     
+ Misses       192     188    -4     

Files with missing lines	Coverage Δ
message_ix/__init__.py	100.0% <100.0%> (+8.3%)	⬆️
message_ix/core.py	97.7% <100.0%> (ø)
message_ix/macro.py	96.7% <100.0%> (ø)
message_ix/report/__init__.py	100.0% <100.0%> (ø)
message_ix/report/operator.py	97.5% <100.0%> (ø)
message_ix/report/pyam.py	100.0% <100.0%> (ø)
message_ix/testing/__init__.py	99.6% <100.0%> (ø)
message_ix/tests/report/test_operator.py	100.0% <100.0%> (ø)
message_ix/tests/test_feature_duration_time.py	100.0% <100.0%> (ø)
..._ix/tests/test_feature_vintage_and_active_years.py	100.0% <100.0%> (ø)
... and 5 more