вариант с "честной" обработкой времени через dateutil
>>> from dateutil.relativedelta import relativedelta
>>> from dateutil.parser import parse
>>> input_iso8601 = '2017-04-22T13:56:43.914375+03:00'
>>> t = parse(input_iso8601)
>>> t
datetime.datetime(2017, 4, 22, 13, 56, 43, 914375, tzinfo=tzoffset(None, 10800))
>>> t2 = t + relativedelta(hour=0, minute=0, second=0, microsecond=0)
>>> t2
datetime.datetime(2017, 4, 22, 0, 0, tzinfo=tzoffset(None, 10800))
>>> t2.isoformat()
'2017-04-22T00:00:00+03:00'