Drop Python 3.9 support

[mr-c]

• Make one more Python 3.9 supporting release when the final release of Python 3.9.x happens at the end of this month (2025-10)
• then merge this PR

[codecov]

Codecov Report

❌ Patch coverage is 83.93393% with 107 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.03%. Comparing base (22bc7d8) to head (cb4eb95).

Files with missing lines	Patch %	Lines
cwltool/cwlprov/provenance_profile.py	34.92%	39 Missing and 2 partials ⚠️
cwltool/main.py	61.22%	11 Missing and 8 partials ⚠️
cwltool/subgraph.py	66.66%	4 Missing and 7 partials ⚠️
cwltool/process.py	92.85%	3 Missing and 2 partials ⚠️
cwltool/secrets.py	78.26%	0 Missing and 5 partials ⚠️
cwltool/workflow.py	66.66%	4 Missing and 1 partial ⚠️
cwltool/load_tool.py	92.45%	2 Missing and 2 partials ⚠️
cwltool/validate_js.py	84.61%	2 Missing and 2 partials ⚠️
cwltool/checker.py	88.00%	2 Missing and 1 partial ⚠️
cwltool/builder.py	95.65%	1 Missing and 1 partial ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2159      +/-   ##
==========================================
- Coverage   85.06%   85.03%   -0.04%     
==========================================
  Files          46       46              
  Lines        8332     8358      +26     
  Branches     1954     1951       -3     
==========================================
+ Hits         7088     7107      +19     
- Misses        773      784      +11     
+ Partials      471      467       -4     

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