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_utildefines.hh

@@ -0,0 +1,42 @@
+/* SPDX-FileCopyrightText: 2023 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+/** \file
+ * \ingroup pygen
+ * \brief header-only utilities
+ * \note light addition to Python.h, use py_capi_utils.hh for larger features.
+ */
+
+#pragma once
+
+#define PyTuple_SET_ITEMS(op_arg, ...) \
+  { \
+    PyTupleObject *op = (PyTupleObject *)op_arg; \
+    PyObject **ob_items = op->ob_item; \
+    CHECK_TYPE_ANY(op_arg, PyObject *, PyTupleObject *); \
+    BLI_assert(VA_NARGS_COUNT(__VA_ARGS__) == PyTuple_GET_SIZE(op)); \
+    ARRAY_SET_ITEMS(ob_items, __VA_ARGS__); \
+  } \
+  (void)0
+
+/**
+ * Wrap #Py_INCREF & return the result,
+ * use sparingly to avoid comma operator or temp var assignment.
+ */
+Py_LOCAL_INLINE(PyObject *) Py_INCREF_RET(PyObject *op)
+{
+  Py_INCREF(op);
+  return op;
+}
+
+/**
+ * Append & transfer ownership to the list,
+ * avoids inline #Py_DECREF all over (which is quite a large macro).
+ */
+Py_LOCAL_INLINE(int) PyList_APPEND(PyObject *op, PyObject *v)
+{
+  int ret = PyList_Append(op, v);
+  Py_DecRef(v);
+  return ret;
+}