lenskit doctor: detect free-threaded Python

[mdekstrand]

This adds detection of Python free-threading support and status to the lenskit doctor command, the first step for #916.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.10%. Comparing base (6ccdfa5) to head (2cb97a0).

Files with missing lines	Patch %	Lines
src/lenskit/cli/doctor.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #996      +/-   ##
==========================================
- Coverage   89.11%   89.10%   -0.02%     
==========================================
  Files         218      218              
  Lines       15015    15021       +6     
==========================================
+ Hits        13381    13384       +3     
- Misses       1634     1637       +3     

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