Self-contained AppImage Pythons for Sigil plugin use on Linux

Because creating self-contained Python AppImages (with all primary Sigil plugin dependencies preinstalled) turns out to be much easier than explaining to the average user how to get around PEP 668 with scary sounding flags, or teaching each and every one of them how to "properly" create virtual Pythons suitable for use with Sigil's plugin framework. Yuck.