From some discussion on the reference implementation PR, it seems that this may be a more complicated feature than I had bargained for: https://github.com/pganssle/zoneinfo/pull/60

The issue is that the current implementation picks up the posix/ and right/ directories as well as the posixrules file, none of which is *wrong* — they are available time zones, after all — but they're not really canonical zones. In `tzdata` we have a list of zones sourced from tzdata's `make zonenames`, which does not include the posix/ and right/ directories.

There is `zone1970.tab` and `zone.tab`, but that has *too* strict of a subset — it only includes canonical zones associated with a specific country, as far as I can tell, and not things like UTC. It also includes, for example, America/Nuuk but not America/Godthab (which was the canonical name up until 2020a).

I'm considering postponing this feature to Python 3.10 so that we can have more time to figure out a decent API for this.

Łukasz: Question for you as release manager — would you prefer that we put this in for 3.9 with the understanding that before the final release we may end up needing to remove it as a nuisance or change what it returns (and possibly flags that would be passed to it), or would that be too disruptive in the beta period? I'm confident that we can make a final decision before October, just not confident that we can make a final decision before Monday.