Support pip 19+

[rmfitzpatrick]

pip 19+ has removed dependency link support: https://pip.pypa.io/en/stable/news/#id4

These changes continue adopting the paths suggested in pypa/pip#4187 (for which I had lost the timeline) and move to full PEP 508 usage with the versioned egg approach for environments with pip 18+. They should be transparent to pip 10 users where --process-dependency-links is still needed.

Adding test coverage for this put tox to its limit w/ configurability, even when considering the perfectly timed https://github.com/pglass/tox-pip-version. These changes move our tox adoption to nox, which proved easier to use with minor performance impacts.

[Jamim]

There are too many bootraps in these changes 😄
(st letters are missing in bootstrap words)

Here is a corresponding PR: #19