Make tests run with Python 2 as well

[Cito]

SQLAlchemy expects new-style classes.

So the manually mapped class must inherit from object, even though this is not necessary in Python 3.

[coveralls]

Coverage increased (+0.4%) to 88.421% when pulling 1def397 on Cito:declarative_issue into c2da407 on graphql-python:master.

[coveralls]

Coverage increased (+0.4%) to 88.421% when pulling 18bd114 on Cito:declarative_issue into c2da407 on graphql-python:master.

[coveralls]

Coverage increased (+0.4%) to 88.421% when pulling 47941dc on Cito:declarative_issue into c2da407 on graphql-python:master.

[Cito]

Sorry, the Travis config was not the problem. Closing this PR to create a better one.