Changes

• Improve enum compatibility (#1153)
• Rename variables called type to type_ (#1216)
• Fix ObjectType meta arguments (#1219)
• Add Base64 scalar (#1221)
• Pin graphql-core to >v3.1.2

Full changelog: https://github.com/graphql-python/graphene-django/compare/v3.0.0b3...v3.0.0b4