Updates to core.py for newer python versions

[Spunky17]

For your consideration.

I forked the docker-headphones to update it as it was deprecated on linuxserver.io. In doing so I also updated it to alpine 3.21 which is considered the standard for docker images these days. In doing so I found that the core.py application produced the errors below. I believe it's a simple fix to add an extra escape character (), while not as clean as some others have suggested it does not cause issues further down the line.

2025-02-13 13:33:03.308390+00:00/app/headphones/lib/gntp/core.py:22: SyntaxWarning: invalid escape sequence '\d'
2025-02-13 13:33:03.308497+00:00'GNTP/(?P<version>\d+\.\d+) (?P<messagetype>REGISTER|NOTIFY|SUBSCRIBE|\-OK|\-ERROR)' +
2025-02-13 13:33:03.308524+00:00/app/headphones/lib/gntp/core.py:29: SyntaxWarning: invalid escape sequence '\d'
2025-02-13 13:33:03.308545+00:00'GNTP/(?P<version>\d+\.\d+) (?P<messagetype>REGISTER|NOTIFY|SUBSCRIBE|\-OK|\-ERROR)',
2025-02-13 13:33:03.308574+00:00/app/headphones/lib/gntp/core.py:33: SyntaxWarning: invalid escape sequence '\w'
2025-02-13 13:33:03.308594+00:00GNTP_HEADER = re.compile('([\w-]+):(.+)')

After making these updates the errors are no longer present when the application starts up.