text: Title: Invalid SQLAlchemy Model
  Body: Hey guys I hope someone can help me on this, i'm really stuck....
  
  database automaping:
  ```
  engine = create_engine(" blablabla", convert_unicode=True)
  
  db_session = scoped_session(sessionmaker(autocommit=False,
                                           autoflush=False,
                                           bind=engine))
  
  Base = automap_base()
  Base.prepare(engine, reflect=True, generate_relationship=_gen_relationship, name_for_scalar_relationship = name_for_scalar_relationship, classname_for_table=camelize_classname, name_for_collection_relationship=pluralize_collection)
  
  Buyer = Base.classes.Buyer
  
  Base.query = db_session.query_property()
  ```
  
  schema : 
  ```
  import graphene
  import json
  from graphene import relay
  from graphene_sqlalchemy import SQLAlchemyConnectionField, SQLAlchemyObjectType
  
  from database import Buyer as BuyerModel
  
  class Buyer(SQLAlchemyObjectType):
  
      class Meta:
          model = BuyerModel
          interfaces = (relay.Node, )
  
  class Query(graphene.ObjectType):
      node = relay.Node.Field()
      all_buyers = SQLAlchemyConnectionField(Buyer)
  
  schema = graphene.Schema(query=Query)
  ```
  
  Produces the error : 
  `AssertionError: You need to pass a valid SQLAlchemy Model in Buyer.Meta, received "<class 'sqlalchemy.ext.automap.Buyer'>".`
  
  