New port: devel/py-pymtbl

pymtbl: Python bindings for the mtbl sorted string table library

pymtbl provides a simple Pythonic wrapper for mtbl's reader, writer,
sorter, and merger interfaces. The examples/ directory contains
scripts demonstrating each of these interfaces.

Differential Revision: https://reviews.freebsd.org/D2220
Approved by: mat (mentor)
Sponsored by: Farsight Security, Inc.