MarsSeed [1] filed a deletion request for python-demjson [2]: Discontinued since 2015. [a] I don't think it's useful to keep.
There's a drop-in continuation fork, demjson3, [b], available on AUR. It provides 'python-demjson' virtually. [c] [a]: https://pypi.org/project/demjson/ [b]: https://pypi.org/project/demjson3/ [c]: https://aur.archlinux.org/packages/python-demjson3 [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/python-demjson/
