The custom readline python module is getting built into the lib dir but should be built into the python module library dir. There is a comment I added way back when I added the module (two FIXMES, actually) in the CMakeLists.txt for the cmake build of the module. Get that resolved - when done, the readline shared object should live in the lib*/python{maj}.{min}/... directory, not the lib directory.
We don't build the readline module anymore. Removed the conditional check in r367956.