More information: readline.read_init_file() produced the same traceback before this commit, so all that's changed is that we're now calling this at interpreter startup. It looks like I'm using the system libedit:
iwasawa:cpython mdickinson$ otool -L build/lib.macosx-10.6-x86_64-3.4-pydebug/readline.so
build/lib.macosx-10.6-x86_64-3.4-pydebug/readline.so:
/usr/lib/libedit.2.dylib (compatibility version 2.0.0, current version 2.11.0)
/usr/lib/libncurses.5.4.dylib (compatibility version 5.4.0, current version 5.4.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.2.11)
iwasawa:cpython mdickinson$