build macOS .app bundles

pyinstaller --windowed is all it takes to make a .app bundle on macOS!

TODO

• learn enough about pyinstaller to figure how to to incorporate that into the .spec
  • needed a new bundle section
• .app bundle is inside the artifacts.zip
• the .app dir seems to contain the same .bin as before: maybe we don't need both.
• test 11 on macOS 11
• test 10.13 on macOS 11
• .app is a directory. how is one supposed to distribute it? Inside a zip? What is a .dmg file?
• changelog entry