fix: python 3 compatibility

jsignature/fields.py

@@ -6,17 +6,18 @@ 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(models.Field):
+class JSignatureField(with_metaclass(models.SubfieldBase, models.Field)):
     """
     A model field handling a signature captured with jSignature
     """
     description = "A signature captured with jSignature"
-    __metaclass__ = models.SubfieldBase
 
     def get_internal_type(self):
         return 'TextField'