Import the psycopg2.extras module fully.

The psycopg2 package import alone won't load the extras module,
resulting in an AttributeError here, when running test cases or other
code that did not previously have the side effect of importing the
extras module.

uuidfield/fields.py

@@ -6,7 +6,7 @@
 
 try:
     # psycopg2 needs us to register the uuid type
-    import psycopg2
+    import psycopg2.extras
     psycopg2.extras.register_uuid()
 except (ImportError, AttributeError):
     pass