py3 compatibility of basestring checks

[kristoff-it]

aliased ‘basestring’ with ‘str’ for py3 compatibility

[lauxley]

Hi, thx for the PR
I'm a little bit confused as to why merge this in feature/django-19 which is not py3 compatible ?
Also this is not the preferred way to deal with basestring according to django's documentation https://docs.djangoproject.com/en/1.9/topics/python3/#string-handling-with-six

isinstance(myvalue, six.string_types)       # replacement for basestring