The solution for me was to force link the readline 8 lib that I had under /usr/local/opt/readline/lib/
.
> cd /usr/local/opt/readline/lib/
> ls
libhistory.8.0.dylib libhistory.a libreadline.8.dylib libreadline.dylib
libhistory.8.dylib libhistory.dylib libreadline.8.0.dylib libreadline.a pkgconfig
Since I have a newer version of the readline lib, I tried to force it to find the one I have by symlinking it to the filename it was looking for. e.g.
> cd /usr/local/opt/readline/lib/
> ln -s libreadline.8.0.dylib libreadline.7.dylib
> ls
libhistory.8.0.dylib libhistory.a libreadline.7.dylib libreadline.8.dylib libreadline.dylib
libhistory.8.dylib libhistory.dylib libreadline.8.0.dylib libreadline.a pkgconfig
pyenv install x.y.z
should now be working (or at least throw a different error ;).