stick to Python's style guide best practice

'Programs must be written for people to read, and only incidentally for
machines to execute.' (Harold Abelson)

http://legacy.python.org/dev/peps/pep-0008/#programming-recommendations

Signed-off-by: Arjun Sreedharan <arjun024@gmail.com>

youtube_dl/YoutubeDL.py

@@ -849,7 +849,7 @@ class YoutubeDL(object):
         # Keep for backwards compatibility
         info_dict['stitle'] = info_dict['title']
 
-        if not 'format' in info_dict:
+        if 'format' not in info_dict:
             info_dict['format'] = info_dict['ext']
 
         reason = self._match_entry(info_dict)