[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/cache.py

@@ -8,10 +8,8 @@ import re
 import shutil
 import traceback
 
-from .utils import (
-    compat_expanduser,
-    write_json_file,
-)
+from .compat import compat_expanduser
+from .utils import write_json_file
 
 
 class Cache(object):