Merge pull request #11015 from tchaikov/wip-optional-py3

cmake: make py3 a nice-to-have

Reviewed-by: Willem Jan Withagen <wjw@digiware.nl>
Reviewed-by: Josh Durgin <jdurgin@redhat.com>

diff --cc ceph.spec.in
Simple merge

diff --cc src/CMakeLists.txt
Simple merge

diff --cc src/pybind/CMakeLists.txt
Simple merge