[python] Convert unicode to six.u (#6881)

`unicode` does not exist in py3, therefore since six is already imported
switch to using `six.u` so that `unicode` is used on py2 and `str` is used
on py3.

samples/client/petstore/python-asyncio/petstore_api/api_client.py

@@ -541,7 +541,7 @@ class ApiClient(object):
         try:
             return klass(data)
         except UnicodeEncodeError:
-            return unicode(data)  # noqa: F821
+            return six.u(data)
         except TypeError:
             return data