Add unittest to exercise python compatibility

It was recently discovered that the IM module is not Python3 compatible
due to issues with the pyangbind library. This patch adds a specific
test that will exercise basic parsing of a VNFD, which passes under
Python2 but fails under Python3. This will be useful for verifying
Python3 support when it lands in pyangbind.

Signed-off-by: Adam Israel <adam.israel@canonical.com>
diff --git a/tox.ini b/tox.ini
index 4493390..c1c0215 100644
--- a/tox.ini
+++ b/tox.ini
@@ -5,6 +5,7 @@
 [testenv]
 deps=nose
      mock
+     pyyaml
 commands=nosetests
 
 [testenv:flake8]