Port some tests to pytest framework (step 2)

[skirpichev]

see https://github.com/aleaxit/gmpy/issues/377

a continuation of #408

[codecov-commenter]

Codecov Report

Merging #437 (b601934) into master (175fa58) will decrease coverage by 0.01%. The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master     #437      +/-   ##
==========================================
- Coverage   83.57%   83.56%   -0.01%     
==========================================
  Files          49       49              
  Lines       11715    11715              
  Branches     2190     2190              
==========================================
- Hits         9791     9790       -1     
- Misses       1924     1925       +1     

see 2 files with indirect coverage changes

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[casevh]

Thanks.