While looping to get all branches of a project, the loop works fine but after sometime i am getting below error. What is main reason for getting the error. How it can be resolved

Traceback (most recent call last):
File "C:\Users\diljale\AppData\Local\Programs\Python\Python36-32\lib\site-packages\gitlab\exceptions.py", line 247, in wrapped_f
return f(*args, **kwargs)
File "C:\Users\diljale\AppData\Local\Programs\Python\Python36-32\lib\site-packages\gitlab\mixins.py", line 94, in list
obj = self.gitlab.http_list(path, **kwargs)
File "C:\Users\diljale\AppData\Local\Programs\Python\Python36-32\lib\site-packages\gitlab_init_.py", line 761, in http_list
return list(GitlabList(self, url, query_data, **kwargs))
File "C:\Users\diljale\AppData\Local\Programs\Python\Python36-32\lib\site-packages\gitlab_init_.py", line 853, in init
self.query(url, query_data, **kwargs)
File "C:\Users\diljale\AppData\Local\Programs\Python\Python36-32\lib\site-packages\gitlab_init.py", line 858, in query
**kwargs)
File "C:\Users\diljale\AppData\Local\Programs\Python\Python36-32\lib\site-packages\gitlab_init.py", line 699, in http_request
response_body=result.content)
gitlab.exceptions.GitlabHttpError: 403: 403 Forbidden

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "<pyshell#34>", line 2, in
for branch in project.branches.list(all=True):
File "C:\Users\diljale\AppData\Local\Programs\Python\Python36-32\lib\site-packages\gitlab\exceptions.py", line 249, in wrapped_f
raise error(e.error_message, e.response_code, e.response_body)
gitlab.exceptions.GitlabListError: 403: 403 Forbidden