If a python-level class defined the __buffer__ attribute, frombuffer(buf, ...) would assume it returns the instance as a buffer. This was undocumented and flaky: if the
__buffer__ attribute was a method it would not be called. I tried to search for where this was ever used and got back to the original numeric code, where once it was a method to be called, but elsewhere an attribute. This second use was preserved until today.

Since it is undocumented I do not think we need a deprecation cycle. Should this hit the mailing list?