FYI, that invariant failed for me just now under the released 3.4.3 too:
Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 24 2015, 22:44:40) [MSC v.1600 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from datetime import *
>>> E = datetime(1970,1,1,tzinfo=timezone.utc)
>>> s = -1/2**7
>>> datetime.fromtimestamp(s, timezone.utc)
datetime.datetime(1969, 12, 31, 23, 59, 59, 992187, tzinfo=datetime.timezone.utc)
>>> E + timedelta(seconds=s)
datetime.datetime(1969, 12, 31, 23, 59, 59, 992188, tzinfo=datetime.timezone.utc)
It's an exactly-tied rounding case for the exactly-representable-in-binary s = -0.0078125.