non-string dict keys in django-celery configs

This fix allows celery-flower to not have problems displaying
configurations for projects that still use configurations embedded
in django settings files.  In this instance there are some int
dict keys that are totally unrelated to celery but that are causing
frequent error messages in the celeryd logs.

celery/worker/control.py

@@ -14,7 +14,7 @@ import tempfile
 from kombu.utils.encoding import safe_repr
 
 from celery.exceptions import WorkerShutdown
-from celery.five import UserDict, items
+from celery.five import UserDict, items, string_t
 from celery.platforms import signals as _signals
 from celery.utils import timeutils
 from celery.utils.functional import maybe_list
@@ -364,7 +364,7 @@ def active_queues(state):
 
 
 def _wanted_config_key(key):
-    return key.isupper() and not key.startswith('__')
+    return isinstance(key, string_t) and key.isupper() and not key.startswith('__')
 
 
 @Panel.register