python: Add test for python3 and fix found problems.

test3/test_except.py

@@ -0,0 +1,14 @@
+#!/usr/bin/env python
+# coding: utf-8
+
+from nose.tools import *
+from msgpack import packs, unpacks
+
+import datetime
+
+def test_raise_on_find_unsupported_value():
+    assert_raises(TypeError, packs, datetime.datetime.now())
+
+if __name__ == '__main__':
+    from nose import main
+    main()