Add testing and docs for supported Python versions

[GumpacG]

Summary

Run gremlin-python tests across all supported Python versions in parallel

Converts the single-version python job in build-test.yml into a fail-fast: false matrix covering Python 3.9-3.13, and
parameterizes gremlin-python/docker-compose.yml (image: python:${PYTHON_VERSION:-3.9}) so the matrix value actually reaches theinterpreter inside Docker.

Updated docs to reflect these changes as well.

Design

The python build-test for multiple versions are running in parallel and should take around the same time to complete (~25 min.) as before this change.

Follow up tasks

Merge to 3.8-dev and master changing the lower bound to 3.10 but maintain the upper bound to 3.13

Note

Assisted-by: Devin: Claude Opus 4.7

[Cole-Greer]

Thanks @GumpacG, VOTE +1

I intend to merge as CTR once actions are complete

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.51%. Comparing base (9b46b67) to head (fccd96c).
⚠️ Report is 460 commits behind head on 3.7-dev.

Additional details and impacted files

@@              Coverage Diff              @@
##             3.7-dev    #3421      +/-   ##
=============================================
- Coverage      76.14%   75.51%   -0.64%     
- Complexity     13152    13167      +15     
=============================================
  Files           1084     1092       +8     
  Lines          65160    67191    +2031     
  Branches        7285     7391     +106     
=============================================
+ Hits           49616    50738    +1122     
- Misses         12839    13830     +991     
+ Partials        2705     2623      -82     

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