MLBEDSW-10313: Use "New" Python mode in pybind11

Updating pybind11 to the newest version changed our build to use the
"Classic" pybind11 Python mode. Set PYBIND11_FINDPYTHON ON to use the
"New" Python mode, which was used before the update.

Change-Id: Ia54ef7363dba259778151de920e11959484580a0
Signed-off-by: Rickard Bolin <rickard.bolin@arm.com>