Maintain JSONField in graphene_django.compat module (#1429)

Fixes #1428

This should improve backwards compatibility, fixing issues in downstream
packages (notably graphene-django-cud
tOgg1/graphene-django-cud#109, and also
graphene-django-extras, both of which depended on
`graphene_django.compat.JSONField`).

Co-authored-by: Steven DeMartini <sjdemartini@users.noreply.github.com>

Author: sjdemartini

graphene_django/compat.py

@@ -1,3 +1,9 @@
+# For backwards compatibility, we import JSONField to have it available for import via
+# this compat module (https://github.com/graphql-python/graphene-django/issues/1428).
+# Django's JSONField is available in Django 3.2+ (the minimum version we support)
+from django.db.models import JSONField
+
+
 class MissingType:
     def __init__(self, *args, **kwargs):
         pass