mongodb backend version controller and defaults added.

celery/backends/mongodb.py

@@ -79,6 +79,14 @@ class MongoBackend(BaseBackend):
         self.url = url
 
 
+        # default options according to pymongo version
+        if pymongo.version_tuple >= 3:
+            self.options.setdefault('maxPoolSize', self.max_pool_size)
+        else:
+            self.options.setdefault('max_pool_size', self.max_pool_size)
+            self.options.setdefault('auto_start_request', False)
+
+
         # update conf with mongo uri data, only if uri was given
         if self.url:
             uri_data = pymongo.uri_parser.parse_uri(self.url)