[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/execafterdownload.py

@@ -3,10 +3,8 @@ from __future__ import unicode_literals
 import subprocess
 
 from .common import PostProcessor
-from ..utils import (
-    shlex_quote,
-    PostProcessingError,
-)
+from ..compat import shlex_quote
+from ..utils import PostProcessingError
 
 
 class ExecAfterDownloadPP(PostProcessor):