Add missing attribs to urllib.response.addinfourl (#968)

stdlib/3/urllib/response.pyi

@@ -4,5 +4,8 @@ from typing import BinaryIO, Mapping, Optional
 from types import TracebackType
 
 class addinfourl(BinaryIO):
+    headers = ...  # type: Mapping[str, str]
+    url = ...  # type: str
+    code = ...  # type: int
     def info(self) -> Mapping[str, str]: ...
     def geturl(self) -> str: ...