[NEW PORT]: comms/py-xmodem: XMODEM protocol implementation

Python XMODEM protocol implementation

This is a literal implementation of XMODEM.TXT, XMODEM1K.TXT and
XMODMCRC.TXT, support for YMODEM and ZMODEM is pending. YMODEM should
be fairly easy to implement as it is a hack on top of the XMODEM
protocol using sequence bytes `0x00` for sending file names (and some
meta data).

WWW: https://github.com/tehmaze/xmodem

Reviewed by: koobs
Approved by: koobs (mentor)
Differential Revision: https://reviews.freebsd.org/D18626