call PyObject_GC_UnTrack() in tp_dealloc()

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

Author: y-fujisaki2

CHANGES.rst

@@ -4,6 +4,9 @@ Changelog
 4.5.0 (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>`_.
+
 
 4.4.2 (2017-05-12)
 ------------------

src/Acquisition/_Acquisition.c

@@ -415,6 +415,7 @@ Wrapper_clear(Wrapper *self)
 static void
 Wrapper_dealloc(Wrapper *self)
 {
+    PyObject_GC_UnTrack(OBJECT(self));
     Wrapper_clear(self);
     Py_TYPE(self)->tp_free(OBJECT(self));
 }