datetime.py fix for issue972, with unit test
Created originally on Bitbucket by dripton (David Ripton)
Was already merged in Bitbucket before import, marked as merged by the import user
Source changeset not longer availableat the time of import. Commit listwill look empty
Copying utcfromtimestamp from the CPython3.2 version of datetime.py seems to fix the roundoff errors in time zone calculations.