Migrate to pytest

[kurayami07734]

Brief Summary of Changes

edited the test command in tox.ini and include pytest in setup.py

What does this PR address?

• [ ] GitHub issue ( #368 )
• [x] Refactoring
• [ ] New feature
• [ ] Bug fix
• [ ] Adds more tests

Reviewer, please note:

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I ran the full test suite before pushing the changes and all the tests pass.

[kurayami07734]

How do I include pytest as a dependency? @const-cloudinary

[const-cloudinary]

@kurayami07734 maybe here: https://github.com/cloudinary/pycloudinary/blob/c5160e76ddc1b150e1248eaf4a828d7ee0c26677/pyproject.toml#L52