[util] Move compatibility functions out of util

utils is large enough without these compatibility functions.

Everything that is present in newer versions of Python (i.e. with dev Python it's just an import) goes into compat.py .
Everything else (i.e. youtube-dl-specific helpers) goes into utils.py .

youtube_dl/postprocessor/ffmpeg.py

@@ -1,5 +1,4 @@
 import os
-import re
 import subprocess
 import sys
 import time
@@ -7,8 +6,10 @@ import time
 
 from .common import AudioConversionError, PostProcessor
 
-from ..utils import (
+from ..compat import (
     compat_subprocess_get_DEVNULL,
+)
+from ..utils import (
     encodeArgument,
     encodeFilename,
     get_exe_version,