Confirm support for Python 3.14 and drop 3.9

[khaeru]

Python 3.14 was released 2024-10-07. This PR is to:

• Check/confirm that message_ix is compatible with the new version by updating CI workflows.
  • Update the GitHub ruleset config to require 3.14 jobs to pass, instead of 3.13.
• Check/confirm the existence of compatible releases of upstream dependencies. If those releases are not immediately available, the PR can aid to monitor and track upstream issues.
  • Support Python3.14 ixmp4#209 and further upstream. At first glance (here), there are 116 FAIL and 56 ERROR results due to this. The same mark ixmp4_209 and handling used in Confirm support for Python 3.14 and drop 3.9 ixmp#602 are used to mark these as XFAIL.
• Drop support for Python 3.9, which reached end-of-life (no more updates, even security updates) on 2025-10-07.
  • Remove any code marked as for Python 3.9 compatibility only. Depending on these changes, part or all of the functionality may remain usable with Python 3.9 if users have extreme need.
  • Make any general clean-ups or improvements possible with features available from Python 3.10.
    • Use the type union operator instead of typing.Optional and typing.Union. This is achieved by running ruff check --select F401,UP007,UP045 --fix or similar multiple times, plus some small manual clean-ups. See the ruff docs.
    • Use match/case instead of some if statements with multiple elif blocks.

Analogous to #881.

How to review

• Read the diff,
• Note that the CI checks all pass.

PR checklist

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

After approval, before merge:

• Rebase, dropping TEMPORARY commit(s).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.5%. Comparing base (fa45877) to head (13c5b5b).
⚠️ Report is 9 commits behind head on main.

⚠️ Current head 13c5b5b differs from pull request most recent head 5bd58c3

Please upload reports for the commit 5bd58c3 to get more accurate results.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #985   +/-   ##
=====================================
  Coverage   96.4%   96.5%           
=====================================
  Files         60      60           
  Lines       5138    5155   +17     
=====================================
+ Hits        4957    4975   +18     
+ Misses       181     180    -1     

Files with missing lines	Coverage Δ
message_ix/common.py	100.0% <100.0%> (ø)
message_ix/core.py	97.4% <100.0%> (ø)
message_ix/macro/calibrate.py	96.7% <100.0%> (ø)
message_ix/report/__init__.py	100.0% <100.0%> (ø)
message_ix/report/pyam.py	100.0% <100.0%> (ø)
message_ix/testing/__init__.py	100.0% <100.0%> (ø)
message_ix/tests/model/message/test_cap_comm.py	100.0% <100.0%> (ø)
message_ix/tests/model/test_message.py	100.0% <100.0%> (ø)
message_ix/tests/report/test_operator.py	100.0% <100.0%> (ø)
message_ix/tests/test_core.py	100.0% <100.0%> (ø)
... and 30 more

[glatterf42]

Thanks for these cleanups! I think the test failures are flaky, but I've triggered a rerun to be sure. Since all tests are passing here, I think this PR includes the fixes for our nightly CI failures, so it would be great to merge this today :)

[khaeru]

FAILED message_ix/tests/report/test_operator.py::test_stacked_bar - _tkinter.TclError: Can't find a usable init.tcl in the following directories: 
    {C:\hostedtoolcache\windows\Python\3.12.10\x64\tcl\tcl8.6}

C:/hostedtoolcache/windows/Python/3.12.10/x64/tcl/tcl8.6/init.tcl: couldn't read file "C:/hostedtoolcache/windows/Python/3.12.10/x64/tcl/tcl8.6/init.tcl": No error
couldn't read file "C:/hostedtoolcache/windows/Python/3.12.10/x64/tcl/tcl8.6/init.tcl": No error
    while executing
"source C:/hostedtoolcache/windows/Python/3.12.10/x64/tcl/tcl8.6/init.tcl"
    ("uplevel" body line 1)
    invoked from within
"uplevel #0 [list source $tclfile]"


This probably means that Tcl wasn't installed properly.

I also ran into this in khaeru/genno#179; will cherry-pick and use the same workaround here.

[khaeru]

With that change, the checks all pass (although slowly 🥲). I'll now rebase to drop the TEMPORARY commit, and then merge.