Refactor: Move BPY generic headers to proper C++ ones.

NOTE: `bgl.h` was left unchanged, as it is deprecated code anyway, and
its conversion to C++ does not seem immediately trivial.

source/blender/python/generic/python_compat.hh

@@ -0,0 +1,17 @@
+/* SPDX-FileCopyrightText: 2023 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+/** \file
+ * \ingroup pygen
+ * \brief header-only compatibility defines.
+ */
+
+#pragma once
+
+/* 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