Make python-six an optional dependency

In Ubuntu and Debian we no longer ship Python 2.7, so the translation
module is an unnecessary dependency. In Python 3 the text type is always
'str', so it is not needed. Make the import optional, so that it can
still support Python 2.7 on RHEL and others that ship it.

Author: bluca

bindings/python/gi/overrides/Modulemd.py

@@ -16,7 +16,10 @@
 
 import functools
 
-from six import text_type
+try:
+    from six import text_type
+except ImportError:
+    text_type = str
 from gi.repository import GLib
 
 import datetime