update-nixpkgs: The httpx client changed its public interface

https://github.com/encode/httpx/blob/master/CHANGELOG.md
> The allow_redirects flag is now follow_redirects and defaults to False.

[...]

> The raise_for_status() method will now raise an exception for any responses
> except those with 2xx status codes. Previously only 4xx and 5xx status codes
> would result in an exception.