I've found many ways to get a relocatable version of Python pretty easily. But none are built with --enable-shared. Meaning they're useless to Sigil. Here's a few links to tools that assist with creating the appimage directories (and images). The last seemed very hopeful, but it doesn't provide Python with shared library support. It's still useful for creating relocatable python installations for Linux in general, though.
https://github.com/probonopd/linuxdeployqt
https://github.com/linuxdeploy
https://github.com/niess/python-appimage