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 81.72840% with 74 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.20%. Comparing base (d38605a) to head (404b2a2).
⚠️ Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
schema_salad/makedoc.py	51.28%	11 Missing and 8 partials ⚠️
schema_salad/fetcher.py	63.82%	12 Missing and 5 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	84.84%	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	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #961      +/-   ##
==========================================
- Coverage   83.45%   83.20%   -0.25%     
==========================================
  Files          22       22              
  Lines        4961     4984      +23     
  Branches     1185     1192       +7     
==========================================
+ Hits         4140     4147       +7     
- Misses        544      556      +12     
- Partials      277      281       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.