Fix superset cli for python3 (#1760)

* Fix superset cli for python3

dict.iteritems() has been removed since dict.items() returns an
iterable in python3. Shouldn't be a big deal for python2 to load
all the data into a list.

Fix #1756

* bin/superset: avoid some work when reading config

We don't need to unpack and then pack again a dictionary.

superset/bin/superset

@@ -23,9 +23,9 @@ class GunicornSupersetApplication(gunicorn.app.base.BaseApplication):
         super(GunicornSupersetApplication, self).__init__()
 
     def load_config(self):
-        config = dict([(key, value) for key, value in self.options.iteritems()
-                       if key in self.cfg.settings and value is not None])
-        for key, value in config.iteritems():
+        config = [(key, value) for key, value in self.options.items()
+                       if key in self.cfg.settings and value is not None]
+        for key, value in config:
             self.cfg.set(key.lower(), value)
 
     def load(self):