FIX: Use correct error message serialization

unittests/test_issues.py

@@ -36,4 +36,4 @@ def test_issue_100():
     xml_el = lxml.etree.parse(filename).getroot()
     with raises(TypeError) as exc_info:
         db.common.models._parse_single_xml_element(xml_el)
-    assert "Invalid datatype: List valued properties" in exc_info.value.msg
+    assert "Invalid datatype: List valued properties" in str(exc_info.value)