Use PEP 757 C-API to import/export ints

[skirpichev]

Closes #467

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 88.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.62%. Comparing base (224745b) to head (d55ed44).
Report is 52 commits behind head on master.

Files with missing lines	Patch %	Lines
src/pythoncapi_compat.h	83.01%	9 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #495      +/-   ##
==========================================
+ Coverage   85.52%   85.62%   +0.09%     
==========================================
  Files          50       50              
  Lines       11656    11755      +99     
  Branches     2202     2222      +20     
==========================================
+ Hits         9969    10065      +96     
- Misses       1687     1690       +3     

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[skirpichev]

@casevh, at this point I would appreciate your feedback on capi-workgroup/decisions#35. Proposed API has noticeable speed regression for int->mpz conversion. I'm not sure if this is acceptable for gmpy2.

[skirpichev]

@casevh, this is ready for review

[casevh]

Thanks for all the work on this.