It is an implementation detail that some people need to know, and that is very unlikely to change.  In the pydev thread, Guido said
"
My feeling is that limiting it to strings is fine, but checking those
strings for resembling identifiers is pointless and wasteful."

We occasionally document such things in a 'CPython implementation detail' note.  I don't know the proper markup for these.  At present, I think the note should be in setattr and **kwargs docs.