Fix dict_keys indexing

[tdeni]

s3cmd fails on python3.6 as follows

Traceback (most recent call last):
  File "/usr/bin/s3cmd", line 3627, in <module>
    rc = main()
  File "/usr/bin/s3cmd", line 3524, in main
    rc = cmd_func(args)
  File "/usr/bin/s3cmd", line 2163, in cmd_sync
    return cmd_sync_remote2local(args)
  File "/usr/bin/s3cmd", line 1719, in cmd_sync_remote2local
    _set_local_filename(copy_pairs, destination_base, source_args, dir_cache)
  File "/usr/bin/s3cmd", line 1408, in _set_local_filename
    source_args[0] == remote_list[remote_list.keys()[0]].get(u'object_uri_str', ''):
TypeError: 'dict_keys' object does not support indexing

Fixes:

• Exception when running s3cmd sync #1363