Use six.string_types instead of basestring

There is no global variable "basestring" in Python 3.
"six.string_types" should be used to support both Python 2 and Python 3

Change-Id: I556fef0e98d00ab819e3dea961666b403b927b06
Closes-bug:#1403077
Merge "Use six.string_types instead of basestring"