Fix test_json failure in Python 3 (#1024)

Make sure we open a NamedTemporaryFile in a way that allows writing
strings (not just bytes). In Python 3, string literals are unicode.

src/unity/python/turicreate/test/test_json.py

@@ -276,7 +276,7 @@ def test_malformed_json(self):
   }
  ]
  """
-        with tempfile.NamedTemporaryFile() as f:
+        with tempfile.NamedTemporaryFile('w') as f:
             f.write(out)
             f.flush()