Fix #1275 -- Use natsort 3.2.0.

This makes Python 3 work properly in this case.

There is one more problem solved.  Previously, we used itemgetter(1)
as the key.  This is worthless for us, and breaks if we use
one-character directory names.

cc @ralsina

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>