fix bug in datetime tuple concatenation

On the master version, just doing an s3cmd ls will give this error:

```
Problem: TypeError: can only concatenate tuple (not "UTC") to tuple
S3cmd:   1.5.0-alpha1

Traceback (most recent call last):
  File "/Users/carlo/Projects/datadog/python/bin/s3cmd", line 2037, in <module>
    main()
  File "/Users/carlo/Projects/datadog/python/bin/s3cmd", line 1978, in main
    cmd_func(args)
  File "/Users/carlo/Projects/datadog/python/bin/s3cmd", line 104, in cmd_ls
    subcmd_bucket_list(s3, uri)
  File "/Users/carlo/Projects/datadog/python/bin/s3cmd", line 158, in subcmd_bucket_list
    "timestamp": formatDateTime(object["LastModified"]),
  File "/Users/carlo/Projects/datadog/s3cmd/S3/Utils.py", line 176, in formatDateTime
    dateS3toPython(s3timestamp)[0:6] + (tz))
TypeError: can only concatenate tuple (not "UTC") to tuple
```

It's not doing tuple concatenation because a single element tuple in python is `(elem,)`, not `(elem)`