Port some tests to pytest framework (step 3)

[skirpichev]

see #377

a continuation of https://github.com/aleaxit/gmpy/pull/437

[codecov-commenter]

Codecov Report

Merging #439 (f8d93b7) into master (9a98d34) will increase coverage by 83.57%. The diff coverage is n/a.

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@@             Coverage Diff             @@
##           master     #439       +/-   ##
===========================================
+ Coverage        0   83.57%   +83.57%     
===========================================
  Files           0       49       +49     
  Lines           0    11718    +11718     
  Branches        0     2190     +2190     
===========================================
+ Hits            0     9793     +9793     
- Misses          0     1925     +1925     

see 49 files with indirect coverage changes

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[casevh]

Thanks!