python3-sphinx: upgrade 7.2.6 -> 7.3.7

python3-sphinx: upgrade 7.2.6 -> 7.3.7

License-Update: Update copyright year, add .rst extension

There are many changes brought in by this uprev, in particular with the
major release 7.3.0, which contains many bugfixes and new features. Changelog:
https://www.sphinx-doc.org/en/master/changes.html#release-7-3-7-released-apr-19-2024

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>