Don't include setup_requires in package template

[ngoldbaum]

Currently the template suggests using setup_requires:

https://github.com/audreyr/cookiecutter-pypackage/blob/f1e9c655b56ec1061f8f7bab5a5bdeef0f4da41a/%7B%7Bcookiecutter.project_slug%7D%7D/setup.py#L62

I think this is a footgun, since setup_requires interacts badly with pip. In particular, it uses easy_install to install packages if they're needed. For pytest_runner this is probably harmless but for a package like numpy that normally comes in a wheel but easy_install will try to build from source this is a big deal. I don't think it's good practice to suggest using setup_requires, especially in something like cookiecutter which is aimed at non-expert python users.