Notes for myself.

In bpo-38392, I modified PyObject_GC_Track() in debug mode to detect this bug.

For example, this bug can be reproduced with this change:
---
diff --git a/Python/hamt.c b/Python/hamt.c
index 28b4e1ef6c..d7dd555d15 100644
--- a/Python/hamt.c
+++ b/Python/hamt.c
@@ -2478,8 +2478,6 @@ hamt_alloc(void)
     if (o == NULL) {
         return NULL;
     }
-    o->h_count = 0;
-    o->h_root = NULL;
     o->h_weakreflist = NULL;
     PyObject_GC_Track(o);
     return o;
---

Python now detects the bug in debug mode:
---
$ ./python -m test -v test_context
(...)
test_context_copy_1 (test.test_context.ContextTest) ...
Modules/gcmodule.c:1931: visit_validate: Assertion failed: PyObject_GC_Track() object is not valid
Enable tracemalloc to get the memory block allocation traceback

object address  : 0x7f2b17408d70
object refcount : 1
object type     : 0x76bc20
object type name: hamt
object repr     : <hamt object at 0x7f2b17408d70>

Fatal Python error: _PyObject_AssertFailed
Python runtime state: initialized

Current thread 0x00007f2b25590740 (most recent call first):
  File "/home/vstinner/python/master/Lib/test/test_context.py", line 322 in test_context_copy_1
  File "/home/vstinner/python/master/Lib/unittest/case.py", line 616 in _callTestMethod
  File "/home/vstinner/python/master/Lib/unittest/case.py", line 659 in run
  (...)
  File "/home/vstinner/python/master/Lib/runpy.py", line 192 in _run_module_as_main
Aborted (core dumped)
---