prefer pyrepl to readline

This is mainly to start a discussion, not necessarily a proposal to merge as-is.

avoids various readline-related issues, such as #1286.

By preferring pyrepl to readline, we can remove the gross sys.path hack to get around System libedit readline on OS X.

I don't yet know the full limitations of pyrepl (I imagine it doesn't work on Windows), but I do know it at least doesn't yet support `readline.read_init_file`.