datetime: convert timestamptz to local timezone

[TheJJ]

This fixes MagicStack/asyncpg#1006 :)

[TheJJ]

can this please be merged?

[elprans]

This is technically a backwards-incompatible change as clients might stringify returned datetime objects or otherwise rely on the timezone being UTC. I would play it safe and add a configuration knob to CodecContext.

[TheJJ]

Would you be willing to implement that? Or is there another way of defaulting timezone-including datetime objects?
As the timestamptz does include the timezone, returning it as UTC seems a bug to me.

[eltoder]

This is almost certainly not what you want. astimezone() creates a broken fixed-offset timezone object that does not do DST. This exists because python did not use to have a proper timezones implementation, but it has zoneinfo now. A better way would be to have a timezone object in settings, defaulting to UTC, and do astimezone(settings.timezone) here.