switch whereto doc job to python3
switch whereto doc job to python3

The whereto doc build uses an extension that requires python 3, so
update the job to run under python 3.

Change-Id: I9eaca116349d529723f2d70d5d27571e0675fc91
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack-infra/project-config,openstack-infra/project-config