Fix `TypeError` in Python-3.2

This code may raise

    TypeError: string argument without an encoding

powerline/bindings/zsh/__init__.py

@@ -57,7 +57,7 @@ class Args(object):
 			return None
 		else:
 			if isinstance(ret, (unicode, str, bytes)):
-				return ret.split(type(ret)(':'))
+				return ret.split((b':' if isinstance(ret, bytes) else ':'))
 			else:
 				return ret