[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/atomicparsley.py

@@ -6,10 +6,11 @@ import os
 import subprocess
 
 from .common import PostProcessor
-
+from ..compat import (
+    compat_urlretrieve,
+)
 from ..utils import (
     check_executable,
-    compat_urlretrieve,
     encodeFilename,
     PostProcessingError,
     prepend_extension,

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):

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,

youtube_dl/postprocessor/xattrpp.py

@@ -3,10 +3,12 @@ import subprocess
 import sys
 
 from .common import PostProcessor
+from ..compat import (
+    subprocess_check_output
+)
 from ..utils import (
     check_executable,
     hyphenate_date,
-    subprocess_check_output
 )