Fix: Use default backoff if not specified.

Author: anishathalye

dropbox/dropbox.py

@@ -320,7 +320,7 @@ def request_json_string_with_retry(self,
                     backoff = e.backoff if e.backoff is not None else 5.0
                     self._logger.info(
                         'Ratelimit: Retrying in %.1f seconds.', backoff)
-                    time.sleep(e.backoff)
+                    time.sleep(backoff)
                 else:
                     raise