Generated RFC links use outdated base url #9857

Describe the bug

When RFC references are included in docs, Sphinx inserts links to the relevant IETF page for that RFC. It currently uses https://tools.ietf.org/html/ as the base url, but the tools subdomain is deprecated and RFC documents are now hosted at https://datatracker.ietf.org/doc/html/. Links are already being redirected to the new domain.

Mailing list discussion: https://mailarchive.ietf.org/arch/msg/ietf/0n-6EXEmkTp3Uv_vj-5Vnm3o0bo/

The base url appears to be configured here:

How to Reproduce

$ git clone https://github.com/python/peps
$ cd peps
$ pip install -r requirements.txt
$ make sphinx-local
$ open build/pep-0001.html

The RFC 822 text on that page will link to http://tools.ietf.org/html/rfc822.html. This link then redirects to https://datatracker.ietf.org/doc/html/rfc822.html

Expected behavior

The current IETF subdomain for RFC documents should be used, rather than a deprecated one.

Your project

https://github.com/python/peps

Screenshots

No response

OS

Mac, 11.6.1

Python version

3.9.7

Sphinx version

4.3.0

Sphinx extensions

No response

Extra tools

No response

Additional context

I noticed because of a report about RFC links in PEP documents on the Python distutils mailing list: https://mail.python.org/archives/list/distutils-sig@python.org/thread/RXW2MPUWYUVHERA365JJSBFDQPXGP37M/#RXW2MPUWYUVHERA365JJSBFDQPXGP37M