player: Close ffmpeg stdin during streaming

A `stdin` of `None` means the ffmpeg subprocess input inherits from the
parent process, which may cause undesired control from a terminal (e.g.
`C` causes ffmpeg to prompt for a command).  It also closes the parent's
stdin when the subprocess exits.

This commit switches to `subprocess.DEVNULL`, which provides a separate
pre-closed stdin for ffmpeg subprocesses.

discord/player.py

@@ -203,7 +203,7 @@ class FFmpegPCMAudio(FFmpegAudio):
 
     def __init__(self, source, *, executable='ffmpeg', pipe=False, stderr=None, before_options=None, options=None):
         args = []
-        subprocess_kwargs = {'stdin': source if pipe else None, 'stderr': stderr}
+        subprocess_kwargs = {'stdin': source if pipe else subprocess.DEVNULL, 'stderr': stderr}
 
         if isinstance(before_options, str):
             args.extend(shlex.split(before_options))
@@ -293,7 +293,7 @@ def __init__(self, source, *, bitrate=128, codec=None, executable='ffmpeg',
                  pipe=False, stderr=None, before_options=None, options=None):
 
         args = []
-        subprocess_kwargs = {'stdin': source if pipe else None, 'stderr': stderr}
+        subprocess_kwargs = {'stdin': source if pipe else subprocess.DEVNULL, 'stderr': stderr}
 
         if isinstance(before_options, str):
             args.extend(shlex.split(before_options))