fix: fix annnotation of oauths access_token (#855)

Author: kunihik0

boxsdk/auth/oauth2.py

@@ -53,7 +53,7 @@ def __init__(
             store_tokens: Optional[Callable[[str, str], None]] = None,
             box_device_id: str = '0',
             box_device_name: str = '',
-            access_token: str = None,
+            access_token: Optional[str] = None,
             refresh_token: Optional[str] = None,
             session: Optional[Session] = None,
             refresh_lock: Optional[ContextManager] = None,