I just spent quite some time searching for
documentation on the interface for __reduce__.  Turns
out, it's in the library documentation, 3.14.5.2, under
the heading "Pickling and unpickling extension types."

Perhaps that heading can be changed to more clearly
indicate that "__reduce__ is documented here!"  At
least, when I'm writing new-style classes (which the
pickle/copy machinery doesn't accept anything but a
copy_reg registration or __reduce__ or __reduce_ex__) I
don't generally consider my work to be an "extension
class" and thus hadn't ever looked under that heading.