Replace deprecated clock() method by process_time()

[laurent-girod]

Hello,

Some tests fail because the method time.clock() was removed from Python 3.8.

I replaced the calls to this method by time.process_time() to measure the processing time. Additionally, process_time() should have an identical behavior across OSes.

Best regards,

Laurent Girod

[bamthomas]

Hello, would it be possible to release petlib because as we cannot release with pypi when there is a github commit id, we have all our depencies that remains with github commits.

Thank you,

Best regards