Drop support for Python2

brian.lane raised this question in WT-4815, I thought it deserved more visibility.

I think people in the storage engines group are the primary users of the Python API.  For us, removing python2 support would remove an extra step for anyone making any substantial test changes.  I don't see any downside (for us) to remove support.

I'm aware of a single user outside MongoDB that occasionally submits Python issues, and they are pushing for Python3.  It's possible there are other (silent) Python2 users.