PyPI time machine

PyPI time machine is a cool new tool for freezing PyPI in time, like repo2docker does with CRAN. This might be useful for more reproducible requirements.txt builds. Not sure exactly how to integrate it, but worth considering as it can solve the partial-pin-down problem.

Also for anyone else not magically in both Austin and Basel this week, check out the #scipy2019 and #europython2019 for lots of cool stuff going on.