Tox using pytest-runner setup scripts leads to invalid deps

[sliverc]

Currently tox uses pytest-runner setup script wrapper. This way tests are run differently when run locally as stated in README.md.

This may lead to different dependencies and errors as for example #634

Best to split up requirements as in DRF and remove setup pytest-runner script. Once this is done test_requires in setup.py would not be needed which remoes the duplication to requirements-development.txt