urllib.request.HTTPRedirectHandler fails if there is a 307 HTTP return code in a POST request.

In https://github.com/python/cpython/commit/c379bc5ec9012cf66424ef3d80612cf13ec51006 the same behaviour was implemented for 308.

According to https://datatracker.ietf.org/doc/html/rfc7538#section-3 IMHO this implementation is wrong for 308:

> Note: This status code is similar to 301 (Moved Permanently)
> ([RFC7231], Section 6.4.2), except that it does not allow changing
> the request method from POST to GET.

A similar note exists in https://datatracker.ietf.org/doc/html/rfc7231#section-6.4.7 regarding code 307.

So, as long as the original redirection is in POST and it gets redirected to another POST, it should not be an error.

FWIW, in https://github.com/traefik/traefik/issues/7829#issuecomment-770210080 you can see that Traefik proxy uses 307 and 308 codes by default when redirecting non-GET requests.

Thus, urllib.request.HTTPRedirectHandler will fail to process any POST requests proxied by Traefik.