bpo-40321: Support HTTP response status code 308 in urllib.request (#19588)

* Support HTTP response status code 308 in urllib.

HTTP response status code 308 is defined in https://tools.ietf.org/html/rfc7538 to be the permanent redirect variant of 307 (temporary redirect).

* Update documentation to include http_error_308()

* Add blurb for bpo-40321 fix

Co-authored-by: Roland Crosby <roland@rolandcrosby.com>