doc/sphinx/requirements.txt: Bump certifi up

Upgrade certifi to the latest version, to remove e-Tugra from the root
store.

Link: https://groups.google.com/a/mozilla.org/g/dev-security-policy/c/C-HrP1SEq1A?pli=1


Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>