devel/py-libzfs: Fix lzc_wait and lzc_sync detection

A typo in the configure script caused detection of these functions to
always fail. The build succeeds, but the configuration is incorrect.

While here, python:3.6+ is always the case since 3.6 is the lowest
version supported in the ports tree now.

Reviewed by: lwhsu
Approved by: lwhsu (ports)
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D31396