Fixes a non-deterministic failing test in bigquery.

This has something to do with rounding of floats (and different
behavior between Python 2 and Python 3). For example, `.123` gets
"rounded down" when expressed as floating point:

>>> (0.123).hex()
'0x1.f7ced916872b0p-4'
>>> # Add 4 for exponent and multiply .123 by 1000
>>> 2**(52 + 4) * 123
8863084066665136128
>>> # Account for the factor of 1000 we've multiplied .123 by
>>> (0x1f7ced916872b0) * 1000
8863084066665136000

gcloud/bigquery/_helpers.py

@@ -42,8 +42,12 @@ def _datetime_from_prop(value):
     """
     if value is not None:
         # back-end returns timestamps as milliseconds since the epoch
-        value = datetime.datetime.utcfromtimestamp(value / 1000.0)
-        return value.replace(tzinfo=pytz.utc)
+        seconds = int(value / 1000.0)
+        microseconds = 1000.0 * (value - 1000 * seconds)
+        return (
+            _EPOCH +
+            datetime.timedelta(seconds=seconds, microseconds=microseconds)
+        )
 
 
 def _prop_from_datetime(value):