Remove PyGtk from requirements.txt
Remove PyGtk from requirements.txt

PyGtk is not installable from pypi. By removing it from requirements.txt
we will be able to at least "pip install kiwi-gtk".

PyGtk dependency should be checked anyway when importing it.

Change-Id: I5d5164773c7925d11f268e3e1b586c4166084736

stoq/kiwi