use timeit.default_timer instead of time.clock() / time.time()

Replaced usage of time.clock() on windows as well as time.time() elsewhere
for microsecond timestamps with timeit.default_timer(), as time.clock() is
being removed in Python 3.8.   Pull request courtesy Christoph Reiter.

Fixes: #301

Closes: #302
Pull-request: https://github.com/sqlalchemy/mako/pull/302
Pull-request-sha: a706e952727337702692fd1d8369d259e1600d6e

Change-Id: I24e2cd3d2c02323a6fa2b063e86cabe555df2036