feat: release the GIL

[mayeut]

fix #293

[codecov]

Codecov Report

Merging #300 (05657c4) into master (268c2f4) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage   92.99%   93.11%   +0.11%     
==========================================
  Files           9        9              
  Lines        1071     1089      +18     
  Branches      157      158       +1     
==========================================
+ Hits          996     1014      +18     
  Misses         38       38              
  Partials       37       37              

Impacted Files	Coverage Δ
pybase64/_pybase64.c	86.56% <100.00%> (+0.62%)	⬆️

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