Convert to Python 3: `iter(C).next()` -> `next(iter(C))`

tests/test_hoover.py

                 'p': ['a', 'b'],
                 'd': [False, True]
             }
-            iter(hoover.Cartman(source, scheme)).next()
+            next(iter(hoover.Cartman(source, scheme)))
 
         self.assertRaises(ValueError, fn)
 
                 'p': ['a', 'b'],
                 'd': [False, True]
             }
-            iter(hoover.Cartman(source, scheme)).next()
+            next(iter(hoover.Cartman(source, scheme)))
 
         self.assertRaises(ValueError, fn)
 
                 'c': a_mark
             }
             source = dict.fromkeys(scheme, [])
-            iter(hoover.Cartman(source, scheme)).next()
+            next(iter(hoover.Cartman(source, scheme)))
 
         self.assertRaises(ValueError, fn)