devel/py3-logbook: new Python 3 slave port of devel/py-logbook

[philip]

Summary

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.

Test Plan

Poudriere tested on 10amd64, 11amd64 and 12amd64. Build-tested with the
updated devel/py3-threema-msgapi port

portlint gives the usual errors for slave ports (no surprises).

[lwhsu]

D12464 is committed so I guess we don't need this patch, right?

[philip]

Correct. This is overtaken by events.