[NEW PORT] net/py-sbws: Tor network bandwidth scanner

Simple Bandwidth Scanner (a.k.a sbws) is a Tor bandwidth scanner that produces
bandwidth measurements files to be used by Directory Authorities.

The scanner builds two hop circuits consisting of the relay being measured and
a fast exit. Over these circuits it measures RTT and bandwidth.

WWW: https://sbws.readthedocs.io/

Approved by: araujo (mentor)
Differential Revision: https://reviews.freebsd.org/D17606