Add mpz.__array__() method to interact with numpy

[skirpichev]

Closes #507

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.64%. Comparing base (224745b) to head (5099d35).
Report is 46 commits behind head on master.

Files with missing lines	Patch %	Lines
src/gmpy2_mpz_misc.c	95.23%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
+ Coverage   85.52%   85.64%   +0.11%     
==========================================
  Files          50       50              
  Lines       11656    11697      +41     
  Branches     2202     2213      +11     
==========================================
+ Hits         9969    10018      +49     
+ Misses       1687     1679       -8     

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