TST: make tests pass in py2 and py3

unittests/test_connection.py

@@ -232,10 +232,10 @@ def test_missing_implementation():
 def test_bad_implementation_not_callable():
     connection = configure_connection()
     with raises(ConfigurationException) as exc_info:
-        connection.configure(implementation="str")
+        connection.configure(implementation=None)
     assert exc_info.value.args[0].startswith(
         "Bad CaosDBServerConnection implementation.")
-    assert exc_info.value.args[1].args[0] == "'str' object is not callable"
+    assert exc_info.value.args[1].args[0] == "'NoneType' object is not callable"
 
 
 def test_bad_implementation_wrong_class():