I've looked more into it, the issue is that even before an object can be tested with `isinstance()`, both inspect.classify_class_attrs and in pydoc, classdoc local function `spill()` use a `getattr()` call, which triggers the property.

So I think my PR is going in the right direction, adding guards in the inspect function and in two `spill()` functions. If `isinstance()` can be fixed to detect class properties correctly, these guards can be simplified but they still need to be there, I think.