This new py3-* slave port is needed as a dependency of devel/py3-threema-msgapi.
A hook prevents me from adding new py3-* ports without portmgr approval.
Proposed commit message:
devel/py3-logbook: Python 3 slave port of devel/py-logbook Approved by: portmgr Differential_Revision: https://reviews.freebsd.org/D12740
See also D12657 for another py3-* slave port I need to update devel/py3-threema-msgapi.