PyAPI: support building with the up-coming Python 3.12

- Account for new member in _PyArg_Parser.

- Many Python op-codes have been removed.
  For the moment these are disabled in is_opcode_secure.
  Some should be added back as intrinsics, noted in code-comments.

source/blender/python/generic/python_compat.h

@@ -0,0 +1,25 @@
+/* SPDX-FileCopyrightText: 2023 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+/** \file
+ * \ingroup pygen
+ * \brief header-only compatibility defines.
+ */
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Add `intialized` member for Python 3.12+. */
+#if PY_VERSION_HEX >= 0x030c0000
+#  define PY_ARG_PARSER_HEAD_COMPAT() 0,
+#else
+#  define PY_ARG_PARSER_HEAD_COMPAT()
+#endif
+
+#ifdef __cplusplus
+}
+#endif