Define a stability policy (#2529)

Fixes #2394. Eventually fixes #517.

This is essentially @pradyunsg's suggestion from #2394. I suggest that at the
same time we start the formal stability policy, we take a few other disruptive steps
and drop Python 2 and the "b" marker.

Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>