The attached patch changes the default behavior of ob-python sessions,
to respect python-shell-interpreter(-args) when starting an interactive
session.

It also allows separate customization of the default Python command for
nonsessions and sessions.

This mainly benefits IPython users.  IPython isn't suitable as the
default command for nonsessions, but until now there isn't a way to set
it as the default interpreter for sessions only.

Another benefit is to promote greater consistency between ob-python.el
and upstream python.el, in particular for shells started with
run-python.  If a user configures python-shell-interpreter(-args), then
ob-python will respect those settings now.

As explained in the NEWS entry, this change should have no effect on
users who previously configured `org-babel-python-command', or on users
who stick to the default `python-shell-interpreter'.  But I submit the
patch for review before applying it, because it involves changing the
default values of some custom variables.