add a missing import statement

Add the import inside the function rather than at the top of the file
because otherwise it would introduce a circular dependency.

gitlab/objects.py

@@ -31,6 +31,7 @@
 
 class jsonEncoder(json.JSONEncoder):
     def default(self, obj):
+        from gitlab import Gitlab
         if isinstance(obj, GitlabObject):
             return obj.__dict__
         elif isinstance(obj, Gitlab):