Confirm support for Python 3.13 and drop 3.8

[khaeru]

Python 3.13 will be released 2024-10-01. This PR is to:

• Check/confirm that ixmp is compatible with the new version. This includes 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:
  • [Python][Packaging] Support Python 3.13 and upload wheels apache/arrow#43519
  • Python 3.13 problems jpype-project/jpype#1204 and related
  • Confirm support for Python 3.13 and drop 3.8 khaeru/genno#143 and numba further upstream
  • Python 3.13 manylinux wheels pallets/markupsafe#463
• Drop support for Python 3.8, which will reach end-of-life (no more updates, even security updates) "approximately [during, before, or after] October 2024" per PEP 569.
  • Remove any code marked as for Python 3.8 compatibility only. Depending on these changes, part or all of the functionality may remain usable with Python 3.8 if users have extreme need.
  • Make any general clean-ups or improvements possible with features available from Python 3.9.
    • Type hint using generics, e.g. list[str] instead of from typing import List/List[str].

These changes would then be on main and ready for the next release at any time.

How to review

• 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. N/A
• Update release notes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (c49a115) to head (be9bb90).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #544     +/-   ##
=======================================
- Coverage   98.9%   98.9%   -0.1%     
=======================================
  Files         44      44             
  Lines       4819    4815      -4     
=======================================
- Hits        4770    4766      -4     
  Misses        49      49             

Files with missing lines	Coverage Δ
ixmp/_config.py	96.7% <100.0%> (ø)
ixmp/backend/__init__.py	100.0% <100.0%> (ø)
ixmp/backend/base.py	100.0% <100.0%> (ø)
ixmp/backend/jdbc.py	97.4% <100.0%> (ø)
ixmp/cli.py	100.0% <100.0%> (ø)
ixmp/core/platform.py	98.9% <100.0%> (ø)
ixmp/core/scenario.py	99.6% <100.0%> (ø)
ixmp/core/timeseries.py	98.6% <100.0%> (ø)
ixmp/model/__init__.py	100.0% <100.0%> (ø)
ixmp/model/base.py	100.0% <100.0%> (ø)
... and 6 more