Replace deprecated time.clock() with time.process_time()
Replace deprecated time.clock() with time.process_time()

time.clock() is deprecated since Python 3.3 and no longer available on 
Python 3.8.
See https://docs.python.org/3/whatsnew/3.8.html#api-and-feature-removals

Includes an import guard to maintain Python 2.7 support.

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