That's why I gave up on Mint. The in-place upgrade path between major versions was always awkward and plagued with random breakage.
Sigil 0.9.9 is in
the Bionic repos. There's no real need to build Sigil from scratch if you don't want to. Just "sudo apt-get install" it. Just make sure to install all those recommended python3-* packages listed if you want to have a fully-functional Sigil plugin framework.