PyApp experimental builds

We tried using https://github.com/ofek/pyapp to make some binaries for MacOS, Windows and Linux. These appear to be working as advertised, which is great.

For a "real" (still experimental) release we need:

- does GHA "release" workflow get rid of the .zip container?
- test "upgrade" (i.e. a PyApp release on X, then does "fowl self update" get us to X + 1)
- ...