Fix JSONField import (#1021)

Author: jkimbo

graphene_django/compat.py

@@ -16,6 +16,6 @@ class MissingType(object):
 
 try:
     # JSONField is only available from Django 3.1
-    from django.contrib.fields import JSONField
+    from django.db.models import JSONField
 except ImportError:
     JSONField = MissingType