ceph-dashboard-pull-requests: install python3 packages only

ceph-dashboard-pull-requests: install python3 packages only

since we've moved to ubuntu focal for running "make check", "ceph-dashboard-pull-requests"
and "ceph-pr-api" tests. and ubuntu focal does not package python2 packages
for python-scipy or python-routes. let's just install the python3
packages.

this change is related to nautilus builds. because nautilus is the last
release which supports python2.

Signed-off-by: Kefu Chai <kchai@redhat.com>