doc/sphinx: Bump idna to 3.7

While we unlikely to have an issue with CVE-2024-3651, it is simple
enough to bump our version of idna to receive the fix, do so.

Reported-by: GitHub dependabot
Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>