Move setuptools to testing requirements

[ulgens]

It was defined in testenv.deps for tox, making it a literal testing requirement. The version requirement is copied from pyproject.toml.

This change helps with the local testing, in a similar way to #9817 does. Because setuptools dependency was only defined for tox, the recommended testing method in https://www.django-rest-framework.org/community/contributing/#testing

# Setup the virtual environment
python3 -m venv env
source env/bin/activate
pip install -e .
pip install -r requirements.txt

# Run the tests
./runtests.py

were skipping coreapi related tests.

[browniebroke]

Do we need it at all? I would have thought we could remove it entirely but maybe it's needed by one of our deps?