These tests were passing with TZ=UTC, but under a non-UTC timezone
they were failing:
E       AssertionError: given
E       X = [{'timestamp': datetime.datetime(2020, 1, 1, 0, 0, tzinfo=tzutc()), …}, …]
E           and
E       Y = [{'timestamp': datetime.datetime(2020, 1, 1, 0, 0, tzinfo=tzlocal()), …}, …]
E       X[0]['timestamp'] != Y[0]['timestamp']
With this fix, they pass either way.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>