[Python] Object size is not considered during vtable de-duplication

[larsclausen]

The Python builder implementation does not consider the object size when it does vtable de-duplication. This can lead to an object using a vtable indicating an object size smaller than the actual object.

E.g. in this example both objects will share the same vtable, with the second object being larger than the size in the vtable.

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)

[thejtshow]

it seems this will be true for any set of objects who have the same structure up until the last member -- the size of the members is implicit for all members except the last (or only) one.