Use __setattr__ to set PolyData._hemi dynamically

[user27182]

Upstream as part of pyvista/pyvista#7716, setting dynamic attributes on meshes will no longer be allowed. MNE currently adds a _hemi attribute dynamically, which is causing integration test failures:
https://github.com/pyvista/pyvista/actions/runs/16335129476/job/46145621314?pr=7716#step:15:2380

I'm pretty sure they'll all be resolved by this PR with this change.

There will be a global flag as part of the API to allow setting dynamic attributes, but pending review/approval of pyvista/pyvista#7716, I'm pretty this will be off by default. Until a release is made with the "official" flag/mechanism for enabling this, a workaround for this restriction is to use object.__setattr__.

[wmvanvliet]

can't this be setattr(mesh._polydata, "_hemi", h)? If that works, I would prefer to avoid calling a private dunder method.

[user27182]

No it must use object.__setattr__ to work. The changes upstream explicitly forbids setting new attributes the "normal" way.

This change can be considered temporary in order to get our CI integration tests passing upstream. Once the change upstream is formalized and a new PyVista release is made, there will be a formal API for this, e.g.

- mesh._polydata._hemi = h
+ with pv.allow_new_attributes():
+    mesh._polydata._hemi = h

If you'd like I can add a TODO or open a new issue to track this. But in the interim this is needed for our MNE integration tests to pass.

[wmvanvliet]

Thanks for the clarification. #13330 is a cleaner way to do this, but thanks for putting in the effort.