build: Add check-python26-compat

A check-python26-compat target is added to the new TUIs codebase
Makefile to check the source files for python-2.6 compatability.

python-2.6 is currently used on RHEL6 and CentOS.

Change-Id: Ib2c69079f1135961200225215c9ea944688f8607
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>