Also note that Sigil has not bundled Python 3.4 with its installers for quite some time. Sigil 1.9.30 bundles Python 3.8.3 with its Windows and macOS installers (soon to be Python 3.11.3).
The only thing that mentions Python 3.4 is the label for the external python executable which actually says. "≥Python3.4". Meaning the external Python needs to be a
minimum of Python 3.4.
The Sigil build documentation includes the list of requisite extra Python modules needed for a sane Sigil Plugin Framework. There's even a
test plugin that can be installed to make sure that Python environment (Bundled or external) is set up correctly to run plugins.