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/gogs/lister.py

@@ -135,7 +135,8 @@ class GogsLister(Lister[GogsListerState, GogsListerPage]):
             response = self.http_request(url, params=params)
         except HTTPError as http_error:
             if (
-                http_error.response.status_code == 500
+                http_error.response is not None
+                and http_error.response.status_code == 500
             ):  # Temporary hack for skipping fatal repos (T4423)
                 url_parts = urlparse(url)
                 query: Dict[str, Any] = dict(parse_qsl(url_parts.query))