bug#22663: python-mode: Fix electric indent bug after dedenting colon

[Eli Zaretskii]

> From: Joel Rosdahl <joel@rosdahl.net>
> Date: Thu, 27 Dec 2018 18:38:09 +0100
> 
> It looks like the bug still exists in Emacs 26.1 and on latest master.
> 
> Attached is an updated patch with change log entries in the commit
> message on suggested by
> https://www.gnu.org/software/emacs/manual/html_node/emacs/Sending-Patches.html#Sending-Patches.
> Please also note that the patch includes a test case update that
> verifies that the bug has been fixed.

Thanks, pushed to the master branch.