call PyObject_GC_UnTrack() in tp_dealloc()

see the following sites for details:

* https://bugs.python.org/issue31095
* python/cpython#2974

Fixes #35.

CHANGES.rst

@@ -4,6 +4,9 @@ Changes
 4.1.2 (unreleased)
 ------------------
 
+- Fix the extremely rare potential for a crash when the C extensions
+  are in use. See `issue 35 <https://github.com/zopefoundation/zope.security/issues/35>`_.
+
 - Fix `issue 7
   <https://github.com/zopefoundation/zope.security/issues/7`_: The
   pure-Python proxy didn't propagate ``TypeError`` from ``__repr__``

src/zope/security/_proxy.c

@@ -343,6 +343,7 @@ proxy_clear(SecurityProxy *self)
 static void
 proxy_dealloc(SecurityProxy *self)
 {
+  PyObject_GC_UnTrack((PyObject*)self);
   proxy_clear(self);
   SecurityProxyType.tp_base->tp_dealloc((PyObject*)self);
 }

src/zope/security/_zope_security_checker.c

@@ -368,6 +368,7 @@ Checker_clear(Checker *self)
 static void
 Checker_dealloc(Checker *self)
 {
+  PyObject_GC_UnTrack((PyObject*)self);
   Checker_clear(self);
   Py_TYPE(self)->tp_free((PyObject*)self);
 }