fix: django.utils.six not available before django 1.6

jsignature/fields.py

@@ -4,16 +4,16 @@
 """
 import json
 import six
+
 from django.db import models
 from django.core.exceptions import ValidationError
-from django.utils.six import with_metaclass
 
 from .forms import (
     JSignatureField as JSignatureFormField,
     JSIGNATURE_EMPTY_VALUES)
 
 
-class JSignatureField(with_metaclass(models.SubfieldBase, models.Field)):
+class JSignatureField(six.with_metaclass(models.SubfieldBase, models.Field)):
     """
     A model field handling a signature captured with jSignature
     """