bpo-35229: Deprecate _PyObject_GC_TRACK() (GH-10511)

Deprecate _PyObject_GC_TRACK() and _PyObject_GC_UNTRACK() in the
documentation. These macros are removed from Python 3.8 by the commit
1a6be91.

Author: vstinner

Doc/c-api/gcsupport.rst

@@ -66,6 +66,9 @@ Constructors for container types must conform to two rules:
    A macro version of :c:func:`PyObject_GC_Track`.  It should not be used for
    extension modules.
 
+   .. deprecated:: 3.6
+      This macro is removed from Python 3.8.
+
 Similarly, the deallocator for the object must conform to a similar pair of
 rules:
 
@@ -95,6 +98,9 @@ rules:
    A macro version of :c:func:`PyObject_GC_UnTrack`.  It should not be used for
    extension modules.
 
+   .. deprecated:: 3.6
+      This macro is removed from Python 3.8.
+
 The :c:member:`~PyTypeObject.tp_traverse` handler accepts a function parameter of this type: