Make matplotlib-scalebar dependency optional

[jlaehne]

The matplotlib-scalebar package unfortunately is currently unmaintained, it would therefore be good to make it an optional dependency. For example, the aur package cannot be installed since python 3.12: https://aur.archlinux.org/packages/python-matplotlib-scalebar

[hakonanes]

Thanks, this should be easy to do. Will be part of the next minor release (we're releasing a patch within the next few days).