sphinx: Bump urllib3 version

While not a direct issue for us, urllib3 before 1.26.17 is vulnerable to
CVE-2023-43804 to bump our version up.

Reported-by: GitHub dependabot
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

Use urllib3 2.0.6
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>