198: remove deprecated setuptools' tests_require

[Tusenka]

Remove setuptools' tests_require deprecated method. fixes 198.

• fixes #

Checklist:

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [x] Add or update relevant docs, in the docs folder and in code.
• [ ] Add an entry in docs/CHANGELOG.rst, summarizing the change and linking to the issue.
• [ ] Run pre-commit hooks and fix any issues.
• [x] Run pytest and make sure no tests failed.