[requests] Allow HTTPError.response to be None (#10875)

This aligns with the definition in requests, but means that user code might
need additional assertions to ensure that `HTTPError.response` is not `None`.

stubs/requests/requests/exceptions.pyi

@@ -12,12 +12,7 @@ class RequestException(OSError):
 
 class InvalidJSONError(RequestException): ...
 class JSONDecodeError(InvalidJSONError): ...
-
-class HTTPError(RequestException):
-    request: Request | PreparedRequest | None
-    response: Response
-    def __init__(self, *args: object, request: Request | PreparedRequest | None = ..., response: Response) -> None: ...
-
+class HTTPError(RequestException): ...
 class ConnectionError(RequestException): ...
 class ProxyError(ConnectionError): ...
 class SSLError(ConnectionError): ...