fix: purelib should set py3 tag if unset

[henryiii]

Followup to #655.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 86.44%. Comparing base (160bbca) to head (f7bca40).

Files	Patch %	Lines
src/scikit_build_core/builder/wheel_tag.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #661      +/-   ##
==========================================
- Coverage   86.46%   86.44%   -0.03%     
==========================================
  Files          64       64              
  Lines        3280     3282       +2     
==========================================
+ Hits         2836     2837       +1     
- Misses        444      445       +1     

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