bpo-40241: What's New in Python 3.9: opaque PyGC_Head (GH-20586)

Author: vstinner

Doc/whatsnew/3.9.rst

@@ -1098,6 +1098,10 @@ Porting to Python 3.9
   and refers to a constant string.
   (Contributed by Serhiy Storchaka in :issue:`38650`.)
 
+* The :c:type:`PyGC_Head` structure is now opaque. It is only defined in the
+  internal C API (``pycore_gc.h``).
+  (Contributed by Victor Stinner in :issue:`40241`.)
+
 
 Removed
 -------