Remove broken import BaseQuerySet

import BaseQuerySet never worked as the path was wrong, so
flask-mongoengine would always import QuerySet anyway.

If BaseQuerySet is needed, the import should be fixed using

    from mongoengine.queryset.base import BaseQuerySet

instead of

    from mongoengine.base import BaseQuerySet

Author: lafrech

flask_mongoengine/json.py

@@ -1,10 +1,8 @@
 from flask.json import JSONEncoder
 from bson import json_util
 from mongoengine.base import BaseDocument
-try:
-    from mongoengine.base import BaseQuerySet
-except ImportError as ie: # support mongoengine < 0.7
-    from mongoengine.queryset import QuerySet as BaseQuerySet
+from mongoengine.queryset import QuerySet
+
 
 def _make_encoder(superclass):
     class MongoEngineJSONEncoder(superclass):
@@ -15,7 +13,7 @@ class MongoEngineJSONEncoder(superclass):
         def default(self, obj):
             if isinstance(obj, BaseDocument):
                 return json_util._json_convert(obj.to_mongo())
-            elif isinstance(obj, BaseQuerySet):
+            elif isinstance(obj, QuerySet):
                 return json_util._json_convert(obj.as_pymongo())
             return superclass.default(self, obj)
     return MongoEngineJSONEncoder