[lldb] Remove Python 2 fallback and only support Python 3

This removes the fallback to Python 2 and makes Python 3 the only
supported configuration. This is the first step to fully migrate to
Python 3 over the coming releases as discussed on the mailing list.

http://lists.llvm.org/pipermail/lldb-dev/2020-August/016388.html

As a reminder, for the current release the test suite and the generated
bindings should remain compatible with Python 2.

Differential revision: https://reviews.llvm.org/D85942