Add bounds to sphinx requirement

As ONAP was created before Oct 20 2020 in RTD, we
must set bounds to enforce a compatible Sphinx version
(see manual).

Issue-ID: DOC-782

Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: I858cdbfbaf2fab82216cc392ddcff694ee83e2e6
(cherry picked from commit 86de341d35bd807f588c1b3d7a49353ab9b114bc)
1 file changed