> So you want a third argument that lets you flag if the timezone is DST or not?

The third argument is not a flag, it is a timedelta just like the offset.  I am attaching a python prototype for clarity. (See datetimeex.py.)

Conceptually, a 3-argument timezone is very simple: tzinfo defines three abstract methods: utcoffset(..), tzname(..), and dts(..).  The proposed concrete implementation lets allows the user to provide constant values to be returned from each of these methods.