Make scli package pip-compliant
With this scli can be installed via pip or pipx. As of now the dependency to signal-cli is ignored, meaning that it has to be installed manually beforehand.
QR code support as well as readline have been marked as extras however so that the minimal package does not require them.
I am marking this a a draft as I am not sure if this breaks anything, at least the Arch and Nix packages probably have to be adapted since the main script needed to go into a package directory.
Fixes: https://github.com/isamert/scli/issues/129
Thank you for the config files! It's been possible to add pip support (#129) for a while. But now that there is #224, fixing that would be the project's main priority. Afterward, scli can be added to PyPI. I just have not had the time to devote to it recently..