Actually using altool is deprecated and you get a warning that the uploads will fail after a certain date. Xcode 13+ uses notarytool and will be the only way to submit for notarization in the (near) future. You will need a VM running macOS 11.3 at a minimum to install Xcode 13.
So far I have both methods implemented but just need to clean everything up before I am confident with sharing. This weekend, hopefully.\, as I have other stuff waiting.
M1 users please note that is not that simple (for me or you) to (build
and) share arm64 binaries.
OTOH it's still fine for users to build simpler binaries themselves. No warnings when used on the same build system.