This page (section 3.3.3 of the library reference manual):

http://www.python.org/doc/current/lib/weakref-extension.html#weakref-extension

contains a description of what you need to do to make
your extension types weakrefable.  There are a few
problems with this page.

First, it really belongs in the C API and/or extending
and embedding documentation, not in the library
reference manual.

Second, the page describes having to set
Py_TPFLAGS_HAVE_WEAKREFS in tp_flags, but that's
unnecessary because Py_TPFLAGS_HAVE_WEAKREFS is already
in Py_TPFLAGS_DEFAULT.