Suggestion: Migrate to pytest

[bygu4]

Unit tests in pyformlang are written using unittest framework, however, CI runs these tests with pytest. Of course, pytest supports unittest test cases, but still it looks strange. Maybe it would be better to convert existing tests to pytest ones?

To convert them, we can use utilities like unittest2pytest or pytestify.

Alternatively, we can get rid of pytest, and run tests with unittest instead.

[Aunsiels]

Thank you for the suggestion! I wrote the migration as you suggested and got rid of unittest. See commit 9d576c8f1edd63914e63c34b2d167173b2f566ad .