Sopu is a shitty an unofficial version of the Syncplay client designed for mpv (command-line) and OS X after I ran into a myriad of problems trying to get the official client working. It's built using Twisted for networking and Click for the command-line interface.
Note that the client is not very good and full of shit code. But then again, it works better for me than the actual Syncplay client, so there's at least some value.
Install the client using pip. At no point during development did I think about Python 2.x, so I have zero expectations of it working with it. It's also not a bad idea to isolate the installation into a virtualenv of its own.
pip install git+https://github.com/Hamuko/Sopu
For basic usage, just give Sopu the server address as the only argument, e.g. sopu syncplay.pl:8995. You will be prompted for additional information steps before connecting if not enough is provided via options.
--room TEXT Room to join in the server
--socket TEXT Address for the mpv UNIX socket
--username TEXT Desired username to join server with
--password TEXT Server password
--help Show this message and exit.