Fix MPV socket getting created in CWD

[notpeelz]

Fixes #674

I haven't tested on Windows or MacOS.

[daniel-123]

Overall I think the PR looks quite reasonable with one caveat: the python_mpv_jsonipc.py is technically vendored from https://github.com/iwalton3/python-mpv-jsonipc

It would be best if Syncplay didn't add too much of its own things to jsonipc, but on the other hand I don't think anybody else uses that library to begin with and we already made some changes to it. So it's not like this is a blocker of any kind, just something to be aware of and documented in this PR.

[notpeelz]

I could pass the socket path via the ipc_socket constructor parameter instead and leave the vendored library unmodified.

Edit: I updated the PR. Let me know if there's anything else.

[Et0h]

I have tested in Windows by confirming that mpv works and by using using Power Shell and [System.IO.Directory]::GetFiles("\\.\\pipe\\") to confirm that by default it will create a pipe along the lines of \\.\\pipe\\syncplay-mpv-94244417144407 but that if you add a player argument such as input-ipc-server=\\.\pipe\foobar it will also create a pipe of the specified name.

[Et0h]

Thanks @notpeelz and @daniel-123 for your work on this. Time to accept those changes! :-)