Process pth files even if $PYTHONPATH points to site-packages/

[navytux]

Fixes #1959

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development documentation)!

• ran the linter to address style issues (tox -e fix_lint)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation (not needed)

[codecov]

Codecov Report

Merging #1960 into main will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1960      +/-   ##
==========================================
+ Coverage   93.91%   93.93%   +0.02%     
==========================================
  Files          86       86              
  Lines        4237     4237              
==========================================
+ Hits         3979     3980       +1     
+ Misses        258      257       -1     

Flag	Coverage Δ
#tests	93.93% <ø> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
src/virtualenv/seed/embed/base_embed.py	98.11% <0.00%> (+1.88%)	⬆️

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[navytux]

Thanks