Support Python 3.13

[glatterf42]

Please confirm that this PR has done the following:

• Tests adapted
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

Having just released v0.9.8 of ixmp4, which support Python 3.13, this PR adds support for Python 3.13 to pyam. I adapted the Python badge, some workflow files, the pyproject.toml and the lock file, but please check I haven't forgotten anything.
Please also note that I haven't trialed the PR locally, so if more dependencies don't support Python 3.13 yet, it might still fail.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.0%. Comparing base (ddbb88e) to head (d790263).
Report is 42 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #901    +/-   ##
======================================
  Coverage   95.0%   95.0%            
======================================
  Files         64      66     +2     
  Lines       6134    6387   +253     
======================================
+ Hits        5828    6074   +246     
- Misses       306     313     +7     

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[glatterf42]

I can't re-run just the windows-latest-py3.13 test, but it failed just due to a segfault, so the version support itself should be fine. This is also indicated by the other py3.13 tests passing.

For the legacy test, it failed with this message:

 The current project's supported Python range (>=3.10,<3.14) is not compatible with some of the required packages Python requirement:
  - numpy requires Python <3.13,>=3.9, so it will not be satisfied for Python >=3.13,<3.14

I'm not sure how to handle this. The legacy test always runs with 3.10, but the install step still seems to check the whole version range, which has become incompatible with the bump to Python 3.13. One way around this might be to bump the legacy numpy version used, but that seems to defeat the purpose of the legacy test.

EDIT: I only needed to bump the legacy numpy version from 1.26.0 to 1.26.2, which is acceptable, I hope.

[danielhuppmann]

Looks good, thanks! The badge in the docs is not updated by a PR, but I'll do that in a separate PR on the netcdf docs that I'm working on right now.