util: stop calling os.stat_float_times()
It had Python-wide side effects, and it disappears in 3.7.0.
As of this change, we're mostly working on 3.7.0b2. There are a few
worrying failures, mostly around regular expressions, but we'll have
to tackle those separately.
Differential Revision: https://phab.mercurial-scm.org/D2697
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