Fix Issue #8653 - Python vtables not considering object size

[thejtshow]

Fixes #8653

added object size to the vtable lookup tuple.

Problem output:

>>> import python.flatbuffers as flatbuffers
>>> builder = flatbuffers.Builder()
>>> builder.StartObject(1)
>>> builder.PrependInt32Slot(0, 1, 0)
>>> obj1 = builder.EndObject()
>>>     
>>> builder.StartObject(1)
>>> builder.PrependInt64Slot(0, 2, 0)
>>> obj2 = builder.EndObject()
>>> builder.Finish(obj2)
992
>>> builder.Output()
bytearray(b'\x04\x00\x00\x00\xf2\xff\xff\xff\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x06\x00\x08\x00\x04\x00\x06\x00\x00\x00\x01\x00\x00\x00')

Fixed output:

>>> import python.flatbuffers as flatbuffers
>>> builder = flatbuffers.Builder()
>>> builder.StartObject(1)
>>> builder.PrependInt32Slot(0, 1, 0)
>>> obj1 = builder.EndObject()
>>>     
>>> builder.StartObject(1)
>>> builder.PrependInt64Slot(0, 2, 0)
>>> obj2 = builder.EndObject()
>>> builder.Finish(obj2)
984
>>> builder.Output()
bytearray(b'\x0c\x00\x00\x00\x00\x00\x06\x00\x0e\x00\x04\x00\x06\x00\x00\x00\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x06\x00\x08\x00\x04\x00\x06\x00\x00\x00\x01\x00\x00\x00')

[thejtshow]

@aardappel could I also trouble you to look over this PR? I don't think the repo has a python maintainer listed.