Python 3: Replacing unicode with six.text_type for py3 compatibility

The "unicode" type was renamed to "str" in Python 3.
Use six.text_type to make swiftclient compatible with Python 3.

For more information about changes needed for py3
compatibility, see:
https://wiki.openstack.org/wiki/Python3

Change-Id: Ic65607a69935652a1874340928f626fbcc35c014

swiftclient/multithreading.py

@@ -102,7 +102,7 @@ def get_error_count(self):
     def _print(self, item, stream=None):
         if stream is None:
             stream = self.print_stream
-        if six.PY2 and isinstance(item, unicode):
+        if six.PY2 and isinstance(item, six.text_type):
             item = item.encode('utf8')
         print(item, file=stream)