Hanging when using `exec.session`

[egolep]

I tried using the exec.session tag (which, if I get it correctly, should let me execute different code cells in the same REPL) but the execution always hangs and I'm left with the "wheel" spinning and no output. I've tried it with python, and in my config files I have this section:

["external.exec"] = {
    config = {
        lang_cmds = {
            python = {
                cmd = "python ${0}",
                type = "interpreted",
                repl = "python -i",
            },
        },
    },
},