MarsSeed [1] filed a deletion request for python-m2r [2]: Unneeded, abandoned legacy package. Only python-dephell relied on it, which is discontinued since 2021 and is broken, unbuildable.
The successor module is called m2r2, available on AUR as python-m2r2. [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/python-m2r/
