Switch using np.issubdtype to issubclass for compatibility with np>=1.19.2.
Switch using np.issubdtype to issubclass for compatibility with np>=1.19.2.

PiperOrigin-RevId: 338685883
Change-Id: I05a9e43f5982bb2b6cb2c710a1d6ac098267c35a

tensorflow/agents,tensorflow/agents