DOC: Add a release note for __class_getitem__

Author: Bas van Beek

doc/release/upcoming_changes/19879.new_feature.rst

@@ -0,0 +1,15 @@
+``ndarray``, ``dtype`` and ``number`` are now runtime-subscriptable
+-------------------------------------------------------------------
+Mimicking :pep:`585`, the `~numpy.ndarray`, `~numpy.dtype` and `~numpy.number`
+classes are now subscriptable for python 3.9 and later.
+Consequently, expressions that were previously only allowed in .pyi stub files
+or with the help of ``from __future__ import annotations`` are now also legal
+during runtime.
+
+.. code-block:: python
+
+    >>> import numpy as np
+    >>> from typing import Any
+
+    >>> np.ndarray[Any, np.dtype[np.float64]]
+    numpy.ndarray[typing.Any, numpy.dtype[numpy.float64]]