In 0.4.3, drop support for Python 2

Tor doesn't need Python to build, but several of our tests require Python to run.

Python 2 will reach end of life on Jan 1 2020 (see https://pythonclock.org/). Let's help it out by:

• Making our configure script only detect python 3 as a usable python.
• Removing gross python2 workarounds from our scripts.