New N2VC interface + updated libjuju

This commit introduces the Python3 N2VC module, which acts as a standard
interface to the VCA.

The goal of this is to provide a common way for modules to interface
with the VCA.

- Updated libjuju from 0.6.1 to 0.7.3

Signed-off-by: Adam Israel <adam.israel@canonical.com>
Change-Id: Ide70fb5ae5797eb6486de24653dc09a23f9c009e
diff --git a/modules/libjuju/tests/unit/test_constraints.py b/modules/libjuju/tests/unit/test_constraints.py
index cb9d773..00b9156 100644
--- a/modules/libjuju/tests/unit/test_constraints.py
+++ b/modules/libjuju/tests/unit/test_constraints.py
@@ -6,6 +6,7 @@
 
 from juju import constraints
 
+
 class TestConstraints(unittest.TestCase):
 
     def test_mem_regex(self):