Proper support for the SQLAlchemy Enum type

[Cito]

SQLAlchemy Enums were converted to Graphene Strings by graphene-sqlalchemy so far. This is a proposed patch to convert them into Graphene Enums instead. Note that postgresql.ENUM does not need special casing since it is a subtype of the generic SQLAlchemy Enum. Also note that the patch supports the two variants of defining an SQLAlchemy Enum, namely by using a sequence and by using a Python Enum.

[coveralls]

Coverage increased (+0.2%) to 90.972% when pulling ea98709 on Cito:convert_enums into 08a0072 on graphql-python:master.

[coveralls]

Coverage increased (+0.2%) to 90.972% when pulling 2fac056 on Cito:convert_enums into 08a0072 on graphql-python:master.

[sboisson]

It seems there is a problem if the same Enum is used in several classes, the code attempt to recreate the Enum with the same name..

Also there should be a way to reference the generated graphene.Enum so it can be used in queries and mutation..

[Cito]

@sboisson Thanks for the heads up - I will look into these issues this week.

[Cito]

@sboisson I have now addressed these two problems in #98.

It should now be possible to use the same enum, and you can refer to the enum either using the registry or using sqlalchemy_object_type._meta.fields[column_name].type. You can see how this works in the test_should_query_well unit test.