bug#28713: Inconvenient usage of defconst in python-mode

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@users.sourceforge.net>
> Date: Thu, 05 Oct 2017 19:42:52 -0400
> Cc: 28713@debbugs.gnu.org
> 
> > This is of course a minor hassle, but I wonder if the major mode 
> > could/should
> > use a normal variable (if not a defcustom) instead.
> 
> prettify-symbols-alist is not a defcustom, due to some uncertainty over
> how to produce a reasonable interface for customizing it, if I recall
> correctly.  So I think the python-mode version should not be a defcustom
> either, but I see no reason against making it a normal variable:

Thanks, LGTM.