DOC: Add a note about npt._GenericAlias >=3.11 stability

Author: Bas van Beek

numpy/typing/tests/test_generic_alias.py

@@ -17,6 +17,10 @@
 DType = _GenericAlias(np.dtype, (ScalarType,))
 NDArray = _GenericAlias(np.ndarray, (Any, DType))
 
+# NOTE: The `npt._GenericAlias` *class* isn't quite stable on python >=3.11.
+# This is not a problem during runtime (as it's 3.8-exclusive), but we still
+# need it for the >=3.9 in order to verify its semantics match
+# `types.GenericAlias` replacement. xref numpy/numpy#21526
 if sys.version_info >= (3, 9):
     DType_ref = types.GenericAlias(np.dtype, (ScalarType,))
     NDArray_ref = types.GenericAlias(np.ndarray, (Any, DType_ref))