Timestamps don't always get the right tzinfo attached
Each timezone in pytz has a `_tzinfos` dict:

In [3]: type(pytz.timezone('US/Eastern'))
Out[3]: pytz.tzfile.US/Eastern

In [4]: pytz.timezone('US/Eastern')._tzinfos
Out[4]: 
{(datetime.timedelta(-1, 68400),
  datetime.timedelta(0),
  'EST'): <DstTzInfo 'US/Eastern' EST-1 day, 19:00:00 STD>,
 (datetime.timedelta(-1, 72000),
  datetime.timedelta(0, 3600),
  'EDT'): <DstTzInfo 'US/Eastern' EDT-1 day, 20:00:00 DST>,
 (datetime.timedelta(-1, 72000),
  datetime.timedelta(0, 3600),
  'EPT'): <DstTzInfo 'US/Eastern' EPT-1 day, 20:00:00 DST>,
 (datetime.timedelta(-1, 72000),
  datetime.timedelta(0, 3600),
  'EWT'): <DstTzInfo 'US/Eastern' EWT-1 day, 20:00:00 DST>}

Need some kind of sane (read: performant) way to retrieve the right one of these to attach when boxing a timestamp. Is not as trivial as I thought
