If a USD python hook throws an exception during processing, the USD
stage ptr is "leaked" because the destructor is not called on the
argument class we pass into the python call.
This leak prevents the stage from reaching a reference count of 0 later
and the file is kept open. All attempts to use the same USD file again
will be met with the following error:
`Coding Error: in _CreateNew at line 617 of C:\db\build\S\VS1564R\build\usd\src\external_usd\pxr\usd\sdf\layer.cpp -- A layer already exists with identifier 'path.usda'`
Instead of passing our class into the call normally, which boost::python
would see as needing to make a copy, use a `ref` instead, bypassing the
problem.
Additional details and repro in the PR. Test coverage to follow.
Pull Request: https://projects.blender.org/blender/blender/pulls/126298
f78d3af9c8