devel/py-usbtmc: New port

Python USBTMC provides a pure Python USBTMC driver for controlling
instruments over USB.

It is an alternative to pyvisa, and it has the pulse command
(control_in) which is not implemented in the pyvisa-py backend.

(quote from https://github.com/xyphro/UsbGpib/blob/master/SW/NiceGUI/readme.md)