Python 2.7: Check instance on string_types from six instead of str

Author: Konrad Cempura

openapi_core/schema/schemas/util.py

@@ -1,9 +1,10 @@
 """OpenAPI core schemas util module"""
 from distutils.util import strtobool
+from six import string_types
 
 
 def forcebool(val):
-    if isinstance(val, str):
+    if isinstance(val, string_types):
         val = strtobool(val)
 
     return bool(val)