test_role.py: use string.printable instead of lowercase (fixes Python 3.x)

@@ -21,7 +21,7 @@ from kappa.role import Role
 
 
 def randomword(length):
-    return ''.join(random.choice(string.lowercase) for i in range(length))
+    return ''.join(random.choice(string.printable) for i in range(length))
 
 
 class TestRole(unittest.TestCase):