Filter libpythonXY for greater than single digits

[zackelia]

• Fixed regex for filtering >1 digit for X and Y in libpythonXY
  • I doubt Python 10 is coming anytime soon, but it would work now!
• Added tests for filter_libs in lddtree_external_references

Fixes #418.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (d270db8) 92.33% compared to head (a44c538) 92.33%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #424   +/-   ##
=======================================
  Coverage   92.33%   92.33%           
=======================================
  Files          23       23           
  Lines        1291     1291           
  Branches      302      302           
=======================================
  Hits         1192     1192           
  Misses         57       57           
  Partials       42       42           

Impacted Files	Coverage Δ
src/auditwheel/policy/external_references.py	97.87% <100.00%> (ø)

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