Deallocation of schema

@lelit pointed out that the SchemaDocument is never deallocated, which may be neccesary for some use cases (not all). Implement a `tp_dealloc` method in the python object definition to take care of this.

Locate the Python object definition that owns the SchemaDocument and read how its lifetime is managed. Implement the requested tp_dealloc method there, then verify that SchemaDocument is released when the Python object is deallocated.

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