Your Arch Python is too new. It has dropped support for the imghdr.py module. This was a really stupid decision by the maintainers of python as the module is small but heavily used and should not have been dropped in any point release no matter if deprecated or not earlier.
Luckily, you can extract the pure python imghdr.py file from the previous version of python and add it to the plugin zip archive and all should work.
You can grab it from here:
https://github.com/python/cpython/bl.../Lib/imghdr.py
Since python breaking plugins is not in the best interest for any platform I may add it to the Sigil python plugin standard library if its license is at all compatible.