call PyObject_GC_UnTrack() in tp_dealloc()

    see the following sites for details:
    * https://bugs.python.org/issue31095
    * python/cpython#2974

CHANGES.txt

@@ -4,6 +4,9 @@ Changelog
 2.13.12 (unreleased)
 --------------------
 
+- Fix the extremely rare potential for a crash when the C extensions
+  are in use. See `issue 21 <https://github.com/zopefoundation/Acquisition/issues/21>`_.
+
 
 2.13.11 (2016-09-05)
 --------------------

src/Acquisition/_Acquisition.c

@@ -291,8 +291,9 @@ Wrapper_clear(Wrapper *self)
 static void
 Wrapper_dealloc(Wrapper *self)     
 {
-  Wrapper_clear(self);
-  self->ob_type->tp_free((PyObject*)self);
+    PyObject_GC_UnTrack(OBJECT(self));
+    Wrapper_clear(self);
+    self->ob_type->tp_free((PyObject*)self);
 }
 
 static PyObject *