Replace time.clock with time.perf_counter

[OskarPersson]

time.clock was deprecated in Python 3.3 and removed in 3.8

[mistydemeo]

Unfortunately, time.perf_counter isn't available in Python 2. Is there a reasonable way we can try to accommodate both? Say, at the top among the imports doing:

try:
    from time import perf_counter as clock
except ImportError:
    from time import clock

And then switching all usages to just clock().

Looks like the code using this isn't exercised by CI, which is why CI is passing on Python 2.