Don't pass zero argument to tzinfo.
Don't pass zero argument to tzinfo.

Fixes the following error on pypy;
```
    File "datetime_tz/pytz_abbr.py", line 62, in <module>
      unknown = datetime.tzinfo(0)
TypeError: default __new__ takes no parameters
```

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