[1.5.x] Skipped deprecation warning test on Python 2.6
[1.5.x] Skipped deprecation warning test on Python 2.6

Refs #19546. On Python 2.6, DeprecationWarnings are visible by
default.
Backport of e6949373b from master.

imtapps/django-imt-fork,bliti/django-nonrel-1.5,ccn-2m/django,alx-eu/django,alx-eu/django,hasadna/django,bliti/django-nonrel-1.5,alx-eu/django,alx-eu/django,ccn-2m/django,imtapps/django-imt-fork,hasadna/django,ccn-2m/django,ccn-2m/django,bliti/django-nonrel-1.5,hasadna/django,imtapps/django-imt-fork