Python 3 compatability fix.

PiperOrigin-RevId: 247704781

Author: liyunlu0618

tensorflow_model_optimization/python/core/sparsity/keras/pruning_callbacks.py

@@ -102,7 +102,7 @@ def on_epoch_end(self, batch, logs=None):
     del values[-1]
     del params[-1]
 
-    param_value_pairs = zip(params, values)
+    param_value_pairs = list(zip(params, values))
 
     for mask, mask_value in param_value_pairs[::2]:
       pruning_logs.update({