Ensure HTTPError.response is not None

The implementation of `HTTPError` in `requests` does not guarantee that
the `response` property will always be set. So we need to ensure it is
not `None` before looking for the return code, for example.

This also makes mypy checks pass again, as `types-request` was updated
in 2.31.0.9 to better match this particular aspect. See:
https://github.com/python/typeshed/pull/10875

swh/lister/gitlab/lister.py

@@ -56,6 +56,7 @@ def _if_rate_limited(retry_state) -> bool:
         exc = attempt.exception()
         return (
             isinstance(exc, HTTPError)
+            and exc.response is not None
             and exc.response.status_code == codes.forbidden
             and int(exc.response.headers.get("RateLimit-Remaining", "0")) == 0
         ) or is_retryable_exception(exc)