So I'm not clear what "small change" you're referring to here:

* A change to CPython to reduce the number of false positives by
making this case return False? If that, then no, I don't think a PEP
is needed. But note that user code still can't assume that the above
behaviour couldn't still happen in *other* cases, so the change would
be of limited value (and whether it gets accepted depends on whether
the complexity is justified by the benefit).
* A change to the definition of callable() to remove the possibility
of false positives at all? In that case, yes, a PEP probably *is*
needed, as that's going to affect an awful lot of corner cases, and
will impact all implementations. It's probably not correct to call
this a "small change".
* Something else?