With the help of a quodlibet contributor I discovered that I was missing the right version of icu. Version 71 was installed on my system, but dee seems to require icu 70. I just installed icu70 from AUR and now I get a different error message:
ERROR:../pygobject/gi/gimodule.c:1831:_wrap_pyg_hook_up_vfunc_implementation: assertion failed: (G_TYPE_IS_CLASSED (implementor_gtype))
Bail out! ERROR:../pygobject/gi/gimodule.c:1831:_wrap_pyg_hook_up_vfunc_implementation: assertion failed: (G_TYPE_IS_CLASSED (implementor_gtype))
Maybe some packages need to be rebuilt like in this issue that declension from quodlibet mentioned?
quayadeque-git worked and it looks pretty nice! Might switch to it if issues with quodlibet persist.