Set Ubuntu and python in .readthedocs.yaml

Issue-ID: DOC-811
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: I0c850b2a55ac9d302b28701409e8e1361afea2c7
(cherry picked from commit 1bf96f951e892e11e597c180a099871e32ddba7d)
1 file changed