Based on the UTC/local diagram from the "Mind the Gap" section, am I
correct in thinking that the modified invariant that also covers times
in a gap is:

    dt == datetime.fromtimestamp(dt.astimezone(utc).astimezone(dt.tzinfo).timestamp())

That is, for local times that exist, the invariant "dt ==
dt.astimezone(utc).astimezone(dt.tzinfo)" holds, but for times that
don't exist, "dt.astimezone(utc).astimezone(dt.tzinfo)" will normalise
them to be a time that actually exists in the original time zone, and
that normalisation also effectively happens when calling
"dt.timestamp()".

That can't be right -- There is no way any fromtimestamp() call can return a time in the gap.

I think about the only useful invariant here is

  dt.timestamp() == dt.astimezone(utc).timestamp() == dt.astimezone(<any other tz>).timestamp()