Use .pip/pip.conf to determine, which index to use.

This also affects the use of `yolk3k==0.9`, which has hard-coded  `XML_RPC_SERVER = 'https://pypi.python.org/pypi'` in `.env/lib/python3.7/site-packages/yolk/pypi.py`, which prevents use of the package in some regions. The 'https://pypi.org/pypi'` is generally more available.