Drop dependency on python-six

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.
Simply set the value to 'unicode' on Python2, and 'str' on Python3.

Author: bluca

bindings/python/gi/overrides/Modulemd.py

@@ -16,10 +16,15 @@
 
 import functools
 
-from six import text_type
 from gi.repository import GLib
 
 import datetime
+import sys
+
+if sys.version_info[0] >= 3:
+    text_type = str
+else:
+    text_type = unicode  # noqa: F821
 
 Modulemd = get_introspection_module("Modulemd")