urllib.HTTPError.headers is a settable property (#9682)

stdlib/urllib/error.pyi

@@ -10,7 +10,9 @@ class URLError(IOError):
 
 class HTTPError(URLError, addinfourl):
     @property
-    def headers(self) -> Message: ...  # type: ignore[override]
+    def headers(self) -> Message: ...
+    @headers.setter
+    def headers(self, headers: Message) -> None: ...
     @property
     def reason(self) -> str: ...  # type: ignore[override]
     code: int