Port some tests to pytest framework (step 6, the last)

[skirpichev]

a continuation of https://github.com/aleaxit/gmpy/pull/456

• closes https://github.com/aleaxit/gmpy/issues/377
• document installation from sources and testing, close some old issues: closes https://github.com/aleaxit/gmpy/issues/259, closes https://github.com/aleaxit/gmpy/issues/133, closes https://github.com/aleaxit/gmpy/issues/337

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2183205) 85.05% compared to head (1da53e7) 85.05%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #457 +/- ## ======================================= Coverage 85.05% 85.05% ======================================= Files 49 49 Lines 11733 11733 Branches 2204 2204 ======================================= Hits 9980 9980 Misses 1753 1753 ```

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