schema objects don't retain a class hierarchy

[wllgrnt]

Repro:

from object_database import Schema
subclass_schema = Schema("test_subclassing")

@subclass_schema.define
class BaseClass:
    x = int

@subclass_schema.define
class DerivedClass(BaseClass):
    y = int

assert issubclass(DerivedClass, BaseClass)

This is due to __bases__ and __mro__ not being set properly when the object is created in the define method of schema.py. Options:

1. Implement tp_base in the C
2. Generate a new set_bases() method on the object t that is called during the initialisation process.