resolve django translation deprecation warnings (#847)

https://docs.djangoproject.com/en/3.0/releases/3.0/#id3

Author: luto

graphene_django/forms/forms.py

@@ -2,7 +2,7 @@
 
 from django.core.exceptions import ValidationError
 from django.forms import CharField, Field, MultipleChoiceField
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 from graphql_relay import from_global_id