Support authentication for custom, private PyPI repos

[pombredanne]

Some private PyPI repositories such as devpi or artifactory may support authentication.
We should be able to support that too.
pip supports several ways and we could just adopt these ways

See https://pip.pypa.io/en/stable/topics/authentication/#netrc-support for pip support

[pombredanne]

Let's start with .netrc support that we can extract from pip. See also https://github.com/sloria/tinynetrc and there is also the stdlib netrc module