Fix device flow authorization endpoint

Device authorization endpoint is `device_authorization_endpoint` as per https://www.rfc-editor.org/rfc/rfc8628.html#section-4

src/oic/extension/device_flow.py

@@ -94,7 +94,7 @@ def authorization_request(self, scope=""):
             req["scope"] = scope
 
         http_response = self.host.http_request(
-            self.host.provider_info["device_endpoint"], "POST", req.to_urlencoded()
+            self.host.provider_info["device_authorization_endpoint"], "POST", req.to_urlencoded()
         )
 
         response = self.host.parse_request_response(