[Pkg-emacsen-addons] Bug#899212: Exclusively use Python 3 deps and configure the use of python3 by default

[Nicholas D Steeves]

Package: elpy
Version: 1.20.0-1
Severity: normal

This bug tracks the long/medium-term goal to installing only Python 3
dependencies for Elpy.  Steps of the transition:

1. Configure dh-elpa to `setq elpy-rpc-python-command "python3"`
   - Needed to run self-tests using the Python 3 variants of dependencies.
2. Debug failures, and work with upstream, forwarding patches whenever possible.
3. Add a Debian-specific patch to do #1 automatically in the installed package.
   - At present this must be manually configured, or virtualenvs must be used
4. Finally, drop Python 2 dependencies. Definitely drop them from Build-Depends.
   || retain them as Recommends or Suggests for elpa-elpy?

Please comment, especially on what you'd like to see for #4!

Regards,
Nicholas