Add string type resolution from Python version
Add string type resolution from Python version

Python 2 utilizes basestring. Python 3 removed basestring
and utilizes str. Using the major number of the Python
version, string_type is defined accordingly.

pombredanne/drf-extra-fields,blueyed/drf-extra-fields,Hipo/drf-extra-fields,barseghyanartur/drf-extra-fields,Hipo/drf-extra-fields,ticosax/drf-extra-fields