unnecessary runtime dependency on setuptools

I've noticed that both setup.py and setup.cfg mention setuptools as an install_requires: this makes it a runtime dependency; however from what I can understand setuptoolsis only a dependency at installation time, so it could be listed as a setup_requires instead.

I admit this is just nitpicking that is not likely to have a big impact (as most people will have setuptools installed anyway), but I think it also a pretty harmless change?

Thanks in advance