Fix test for string for python 3
Fix test for string for python 3

basestring does not exist for python 3.

alphagov/digitalmarketplace-buyer-frontend,AusDTO/dto-digitalmarketplace-buyer-frontend,AusDTO/dto-digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,mtekel/digitalmarketplace-buyer-frontend,mtekel/digitalmarketplace-buyer-frontend,mtekel/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,AusDTO/dto-digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,mtekel/digitalmarketplace-buyer-frontend,AusDTO/dto-digitalmarketplace-buyer-frontend