Confirm support for Python 3.13 and drop 3.8

[khaeru]

Python 3.13 will be released 2024-10-01 07. This PR is to:

• Check/confirm that sdmx 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 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 standard collections, e.g. list[str] instead of from typing import List/List[str].

[codecov]

Codecov Report

Attention: Patch coverage is 99.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.90%. Comparing base (5256723) to head (6f113ca).
Report is 101 commits behind head on main.

Files with missing lines	Patch %	Lines
sdmx/tests/test_client.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #195      +/-   ##
==========================================
- Coverage   98.92%   97.90%   -1.03%     
==========================================
  Files          97       97              
  Lines        7839     7841       +2     
==========================================
- Hits         7755     7677      -78     
- Misses         84      164      +80     

Files with missing lines	Coverage Δ
sdmx/client.py	93.98% <100.00%> (-3.83%)	⬇️
sdmx/dictlike.py	97.87% <100.00%> (ø)
sdmx/format/__init__.py	100.00% <100.00%> (ø)
sdmx/format/xml/common.py	100.00% <100.00%> (ø)
sdmx/message.py	100.00% <100.00%> (ø)
sdmx/model/__init__.py	100.00% <100.00%> (ø)
sdmx/model/common.py	99.90% <100.00%> (ø)
sdmx/model/internationalstring.py	100.00% <100.00%> (ø)
sdmx/model/v21.py	100.00% <100.00%> (ø)
sdmx/model/v30.py	100.00% <100.00%> (ø)
... and 24 more

... and 14 files with indirect coverage changes