For the record: Sigil is definitely available in the standard Arch repos. It's in Community.
https://www.archlinux.org/packages/c.../x86_64/sigil/
You may also want to install python-pyqt5 from Arch Extra. More and more Sigil plugins make use of PyQt5.
Python3 is the default on Arch. So its various packages are just python-*, not python3-*.