Thanks for your help but to no avail.
I already use some other Sigil plugins so my Python3 seems to be working elsewhere. I suppressed the path to Python 2.7 and only kept the one for Python3. Is there another plugin that makes use of Python3 so that I can confirm this?
yaourt tells me tcl and tk are already installed.
I left the plugin untouched in its zip container. I donwloaded it again to be sure.