feat: transitive extractor for Python requirements

[cuixq]

This PR enables the transitive extractor for Python requirement files. Considering that there is only the native registry client for PyPI, we use that for requirements.txt transitive extractor.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.79%. Comparing base (6c31336) to head (7500a94).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1877      +/-   ##
==========================================
+ Coverage   65.60%   65.79%   +0.19%     
==========================================
  Files         167      167              
  Lines       16060    16073      +13     
==========================================
+ Hits        10536    10576      +40     
+ Misses       4859     4839      -20     
+ Partials      665      658       -7     

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[another-rex]

#1846 Going to wait for this one to get merged in first, then we need to do some work to support having both extractor types similar to what that PR does for the pomxml extractor.

[cuixq]

@another-rex do you have idea how long this will take roughly?