I agree with Brett. Adding `allow_z` (or perhaps `compact` or `use_utc_designator` if we're bikeshedding) as an optional keyword only argument to `.isoformat()` would allow us to keep the explanation that what `.fromisoformat()` can parse is exactly what `.isoformat()` can produce, while giving a feature to `.fromisoformat()` that many have asked for and that would have minimal overhead in terms of code complexity or runtime performance.

Would you accept a PR that implements that, Paul?