- Zap various occurrences of Python2 in docs and scripts
- Remove six from netlib, and some other places where obvious project-wide
search and replace works.
This should be enough to give us reuse without growing infinitely. This is part
of fixing the memory situation in mitmdump.
TODO: There's an opportunity here for a better algorithm, that expires certs
based on least-recently-accessed time, rather than oldest generated time.
