Debugger always disabled with ipykernel 6.4.0?

Using the latest `6.4.0` version of `ipykernel`, it looks like the debugger is always disabled.



This was first noticed on Binder using this gist: https://gist.github.com/jtpio/6ce26381703355e0ef1da4af742b7f72



But can be reproduced locally:



```bash

mamba create -n tmp python -y

conda activate tmp



# install the latest versions

python -m pip install -U ipykernel debugpy jupyterlab



# start JupyterLab

jupyter lab

```



The spec seems to be installed with `debugger: false` on disk:



```

$ cat ${CONDA_PREFIX}/share/jupyter/kernels/python3/kernel.json

{

 "argv": [

  "python",

  "-m",

  "ipykernel_launcher",

  "-f",

  "{connection_file}"

 ],

 "display_name": "Python 3 (ipykernel)",

 "language": "python",

 "metadata": {

  "debugger": false

 }

}

```



And the kernelspecs return `debugger: false` for `ipykernel` at startup:



![image](https://user-images.githubusercontent.com/591645/132663961-5951a459-963a-49ba-be5c-cd46460b293d.png)



Dependencies:



```bash

$ pip list | grep -E "debugpy|ipykernel"

debugpy             1.4.1

ipykernel           6.4.0

```

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