Offline/behind proxy build can't have setup_requires
I'm having problems with building a package depending on Pandas behind proxy, because setup_requires is specified in setup.py.
"pip install" calls setup.py egg_info, which in turn wants numpy, which it cannot find. Numpy is installed along with Pandas by pip, but setup.py can't know about this.

The same issue would also appear if I'd want install an offline package for the app I'm using.
This is why [pip documentation discourages setup_requires](https://pip.readthedocs.org/en/stable/user_guide/#ensuring-repeatability)

I see that there was a request opposite to mine (#3861) to make it usable with buildout... But maybe there's another way to use it with buildout? I think that pip is more of a Python standard than buildout...
