4.0.0 release checklist

[benoit-pierre]

Pre:

• [x] put Circle CI out of its misery: it will continue trying and failing even though the configuration has been removed...
• [x] update translations (French, Spanish) (for #1165 post release)
• [x] clarify macOS situation: oldest supported version?
• [x] configure PyPI token for release
• [x] update to Python 3.8
• [x] update requirements (taking the macOS situation into account, should address #1183 and #1192)
• [x] review and merge #1153
• [x] review and merge #1149 / #1244 (Save As… feature)
• [x] add Calibre-like warning to plugins manager
• [x] do a pass on setup metadata (changelog link? switch back long description to readme contents?)
• [x] clear 4.0.0 milestone: #904 (libusb), #912 (plugin docs), ~~and #1180 (extensions config)~~ still left
• [ ] create a RC for testing, promote to final when happy
• [x] update AUR (git) package

Post RC/final:

• [ ] update AUR (stable) package
• [x] add Melani plugin to registry (need new version to depend on)
• [ ] add Yawei-Steno3 plugin to registry

Possibly:

• [x] ~update other Linux distributions packages~
• [x] partially merge #999 (punctuation consistency only)
• [x] review and merge #1163 (lock display)
• [x] cherry pick part of #1092
• [x] redesign icon for Big Sur (add dumb white box around it)
• [x] #1067 (register AppImage)

[benoit-pierre]

@morinted: thoughts? Anything I missed?