Remove assertIs for Python 2.6 support.
Remove assertIs for Python 2.6 support.

j5int/python-datetime-tz,j5int/python-datetime-tz,mithro/python-datetime-tz,matthewhampton/python-datetime-tz,davidfraser/python-datetime-tz,davidfraser/python-datetime-tz,mithro/python-datetime-tz