sphinx: Bump urllib3 version

While unlikely to be a direct issue for us, urllib3 before 2.0.7 is
vulnerable to CVE-2023-45803, so bump our version up.

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