[Python-modules-team] Bug#914698: Prompt-toolkit update

[Gordon Ball]

Those that I'm involved with:

* xonsh is compatibile with either prompt_toolkit 1.x or 2.x
* updating jupyter_console -> 6 is blocked on prompt_toolkit 2
* updating ipython -> 7 is blocked on prompt_toolkit 2

The ipython case is complicated because we currently have 5.8 in the
archive, which is the last version supporting 2.7 (and requires
prompt_toolkit 1). Pending a decision on whether to try and keep the
current ipython2.7 version while upgrading the ipython3 version or just
drop the former.

If both versions cannot be supported without needing to maintain two
versions of prompt_toolkit, that probably favours the approach of just
dropping python2 support.