Fixes #1363 - Fixes 'dict_keys' not subscriptable for remote2local with

one local copy

_set_local_filename can be called with copy_paris as remote_list,
and "copy_pairs" is a normal dict and not a FileDict.
So, in that case, copy_pairs.keys() will be a view in py3 and so not
subscriptable.

s3cmd

@@ -1411,10 +1411,10 @@ def cmd_sync_remote2local(args):
             if not os.path.isdir(deunicodise(destination_base)):
                 raise ParameterError("Destination is not an existing directory")
         elif len(remote_list) == 1 and \
-             source_args[0] == remote_list[remote_list.keys()[0]].get(u'object_uri_str', ''):
+             source_args[0] == remote_list[next(iter(remote_list.keys()))].get(u'object_uri_str', ''):
             if os.path.isdir(deunicodise(destination_base)):
                 raise ParameterError("Destination already exists and is a directory")
-            remote_list[remote_list.keys()[0]]['local_filename'] = destination_base
+            remote_list[next(iter(remote_list.keys()))]['local_filename'] = destination_base
             return
 
         if destination_base[-1] != os.path.sep: