Fix support for Unicode value filters

Cinder supports Unicode value filters for API get-pools, therefore,
Here needs to support Unicode value filters as well.

Change-Id: I51df61f9465d84c408fc057443a37834c453de41
Closes-Bug: #1701427

cinderclient/v3/shell.py

@@ -139,7 +139,7 @@ def do_backup_list(cs, args):
            action='store_true',
            help='Show detailed information about pools.')
 @utils.arg('--filters',
-           type=str,
+           type=six.text_type,
            nargs='*',
            start_version='3.33',
            metavar='<key=value>',