Drop Python 3.9 support

[mr-c]

• write more docstrings
• 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 81.90955% with 72 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.11%. Comparing base (deac6ea) to head (975bdfa).

Files with missing lines	Patch %	Lines
schema_salad/fetcher.py	63.04%	12 Missing and 5 partials ⚠️
schema_salad/makedoc.py	54.05%	9 Missing and 8 partials ⚠️
schema_salad/java_codegen.py	77.27%	8 Missing and 2 partials ⚠️
schema_salad/cpp_codegen.py	82.00%	5 Missing and 4 partials ⚠️
schema_salad/dotnet_codegen.py	83.33%	3 Missing and 2 partials ⚠️
schema_salad/python_codegen.py	84.37%	3 Missing and 2 partials ⚠️
schema_salad/typescript_codegen.py	83.33%	3 Missing and 2 partials ⚠️
schema_salad/dlang_codegen.py	91.17%	1 Missing and 2 partials ⚠️
schema_salad/sourceline.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #961      +/-   ##
==========================================
- Coverage   83.37%   83.11%   -0.26%     
==========================================
  Files          22       22              
  Lines        4955     4976      +21     
  Branches     1185     1192       +7     
==========================================
+ Hits         4131     4136       +5     
- Misses        547      559      +12     
- Partials      277      281       +4     

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