Add missing 'headers' property to urllib.error.HTTPError. (#7783)

Seen here: https://docs.python.org/3/library/urllib.error.html#urllib.error.HTTPError.headers

stdlib/urllib/error.pyi

@@ -9,6 +9,8 @@ class URLError(IOError):
     def __init__(self, reason: str | BaseException, filename: str | None = ...) -> None: ...
 
 class HTTPError(URLError, addinfourl):
+    @property
+    def headers(self) -> Message: ...  # type: ignore[override]
     @property
     def reason(self) -> str: ...  # type: ignore[override]
     code: int