__struct_defaults__ is missing from Python type specification

[ncoghlan]

Description

I am attempting to work around #778 by writing a recursive Struct traversal in Python as an alternative to asdict (this is going to be slow, but slow-and-working is better than not working).

From looking at the C code, __struct_defaults__ is available alongside __struct_fields__ (aligning from the end of the two sequences).

However, the Python typing stub for the C code is missing this attribute, so mypy et al complain about accessing the field.