Use call_once to initialize threading

[RainerKuemmerle]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.30%. Comparing base (3687609) to head (efbbd6e).
Report is 1 commits behind head on pymem.

Additional details and impacted files

@@            Coverage Diff             @@
##            pymem     #866      +/-   ##
==========================================
+ Coverage   66.28%   66.30%   +0.02%     
==========================================
  Files         342      342              
  Lines       13405    13409       +4     
  Branches     1320     1320              
==========================================
+ Hits         8885     8891       +6     
+ Misses       4116     4114       -2     
  Partials      404      404              

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