Bring doc building up to python3

Bring doc building up to python3

looks like readthedocs builds have been failing for a while
because python2 can't find some version of sphinx that is
now required by lfdocs-conf. Maybe an older version of
sphinx would still work, but it's time to catch up to
python3 anyway.

integration/test just added this same .readthedocs.yml
file which finally got it's rtd build to work after
having unknowingly been failing for a month:

https://git.opendaylight.org/gerrit/c/integration/test/+/88064
https://readthedocs.org/projects/odl-integrationtest/builds/?page=1

and netconf has been in the same boat:
https://readthedocs.org/projects/odl-netconf/builds/?page=1

Signed-off-by: Jamo Luhrsen <jluhrsen@gmail.com>
Change-Id: I63c01d8d9805ee492ebeb4d1b65723ab8a2d59b9