Why oh why does `pip --no-cache-dir install -vvv .` fail on this?

setup.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import sys
from setuptools import setup


def setup_package():
    # This only works if relaxed to 2.5.6!  Despite all versions up to
    # 2.5.11 being clearly present at https://pypi.org/simple/pyscaffold/
    setup(setup_requires=['pyscaffold>=2.5.10'])


if __name__ == "__main__":
    setup_package()