bpo-38787: C API for module state access from extension methods (PEP 573) (GH-19936)

Module C state is now accessible from C-defined heap type methods (PEP 573).
Patch by Marcel Plch and Petr Viktorin.

Co-authored-by: Marcel Plch <mplch@redhat.com>
Co-authored-by: Victor Stinner <vstinner@python.org>
diff --git a/Include/cpython/object.h b/Include/cpython/object.h
index 45da752..8bf05a3 100644
--- a/Include/cpython/object.h
+++ b/Include/cpython/object.h
@@ -289,6 +289,7 @@
     PyBufferProcs as_buffer;
     PyObject *ht_name, *ht_slots, *ht_qualname;
     struct _dictkeysobject *ht_cached_keys;
+    PyObject *ht_module;
     /* here are optional user slots, followed by the members. */
 } PyHeapTypeObject;