[Datetime-SIG] Trivial vs easy: .utcoffset()

[Tim Peters]

[Tim]
> Easy idea:  after loading a tzfile, create and store (on the tzinfo
> instance) a list of every unique total UTC offset in the zone's
> history, ordered by most recent to least.  .utcoffset() then only
> needs to march through that list once, seeing whether the input minus
> the current offset (from the list) converts back to the input via
> .fromutc().  If none do, that's an internal error.

Not quite.  "Converts back" has to include reproducing the `fold`
value too.  If a user gives a datetime with fold=1 where it doesn't
make sense,.fromutc() can't reproduce it.

Likewise if a user gives a datetime in a gap.

With enough extra annoying code, useful error messages for those could
be produced.  I wouldn't want to _guess_ the intent in either of those
cases.

It's an internal error if neither of those apply.