[prerelease] Removed `instance` attribute on `Module` / `Class`

[The-Compiler]

Yet another subtle API break I discovered in 7.0.0 (sorry...):

Before #9277, Module and Class nodes had a instance attribute from PyobjMixin (via PyCollector). With that change, that attribute now is only available on Function nodes.

This affected pytest-randomly - I don't see a good way to find out how much more it affects (can't really search for .instance, and there are lots of ways to get to those nodes).

I doubt it will affect much more, but we should either re-add the attribute (probably just set to None plus a deprecation warning?), or if we decide to remove it without a deprecation (fine by me), at least document the removal as usual.

cc @bluetech

[bluetech]

Yes, it should be restored (and return None) - assigning to myself.