Misc fixes

[skirpichev]

• more samples + test str(int)==str(mpz) in test_mpz_conversion_bulk()
• more correct error message for MPZ_OVERFLOW error in GMP

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.62%. Comparing base (224745b) to head (71c1186).
Report is 52 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #519      +/-   ##
==========================================
+ Coverage   85.52%   85.62%   +0.10%     
==========================================
  Files          50       50              
  Lines       11656    11704      +48     
  Branches     2202     2213      +11     
==========================================
+ Hits         9969    10022      +53     
+ Misses       1687     1682       -5     

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[casevh]

gmpy used the custom memory allocation feature of GMP to to wrap the malloc calls and raise MemoryError if they failed. It didn't catch all core dumps but this patch looks like a better option. Is there any advantage to raising MemoryError in a future release? (Just having more informative error messages on a core dump is a definite improvement.)