fix late init type variable

Author: AbstractUmbra

mystbin/http.py

@@ -130,6 +130,8 @@ def __init__(self, verb: SupportedHTTPVerb, path: str, **params: Any) -> None:
 
 
 class HTTPClient:
+    root_url: str
+
     __slots__ = (
         "_locks",
         "_owns_session",