[numpy] Fix users of NumPy APIs that are removed in NumPy 2.0.

This change migrates users of APIs removed in NumPy 2.0 to their recommended replacements (https://numpy.org/devdocs/numpy_2_0_migration_guide.html).

PiperOrigin-RevId: 656022757

fedjax/core/serialization.py

@@ -132,7 +132,7 @@ def _msgpack_ext_pack(x):
                            _bytes_ndarray_to_bytes(x))
   elif isinstance(x, (np.ndarray, jax.Array)):
     return msgpack.ExtType(_MsgpackExtType.ndarray, _ndarray_to_bytes(x))
-  elif np.issctype(type(x)):
+  elif isinstance(x, np.generic):
     # pack scalar as ndarray
     return msgpack.ExtType(_MsgpackExtType.npscalar,
                            _ndarray_to_bytes(np.asarray(x)))