Log --debug as UTF-8 to fix charmap mapping

[Et0h]

This should fix errors such as:

File "syncplay\players\mpv.pyc", line 743, in actuallySendLine
File "syncplay\client.pyc", line 1626, in showDebugMessage
builtins.UnicodeEncodeError: 'charmap' codec can't encode character '\u3009' in position 684: character maps to

When using Syncplay with mpv and--debug mode on Windows.

[Et0h]

This seems to fix this issue which I experienced when running Syncplay with mpv when --debug was enabled. If nobody objects I'll merge it soon.

Context: The \u3009 symbol is a Unicode right-angle bracket, and it is the MPV_INPUT_PROMPT_START_CHARACTER specified in constants.py which is passed onto mpv to define the character which appears when you press enter to show you can write your message within mpv.