Every Python developer knows some or all of these libraries, because they’re stable, reliable, and excellent at what they do.

Callipeg Studio heads to macOS and Windows with Inbetween Assist, onion skinning, export routes and one-time pricing.

UPDATE 03/13/2023: This plugin is now natively part of Krita as of Krita 5.0. I will keep this code here for reference, but you don't need to install this plugin any longer to get this functionality.