Replace deprecated is_gpu_available() calls

[Mole1424]

When running foolbox using a tensorflow-based model you get the following error:

WARNING:tensorflow:From .../foolbox/models/tensorflow.py:13: is_gpu_available (from tensorflow.python.framework.test_util) is deprecated and will be removed in a future version.
Instructions for updating:
Use `tf.config.list_physical_devices('GPU')` instead.

this pr implements such suggestion