`breakpoint()` not working

Using a fresh install of Jupyter Lab my usual `breakpoint()` to invoke the debugger in a Notebook is skipped (see screenshot). Using `import pdb; pdb.set_trace()` instead invokes `pdb` as expected.

I was expecting (am I wrong?) that a call to `breakpoint()` in a cell would invoke `pdb`.

This is using a fresh conda install of Jupyter Lab this morning, no reconfiguration of the environment has occurred.

## Description


## Reproduce

I've created a fresh minimal environment:

```
$ conda create -n tmp_env python=3.8 jupyterlab

$ jupyter --version
jupyter core : 4.7.1
jupyter-notebook : 6.4.3
qtconsole : not installed
ipython : 7.27.0
ipykernel : 6.3.1
jupyter client : 7.0.2
jupyter lab : 3.1.10
nbconvert : 6.1.0
ipywidgets : not installed
nbformat : 5.1.3
traitlets : 5.1.0
```

and then I run `jupyter lab` and try to call `breakpoint()` as noted in the screenshot above.

I used `jupyter lab --debug` and scanned the output, nothing looked amiss (but there's a lot and I'm not sure what to look for) - I can attach a copy if needed?

I noticed this originally in a second more complex environment (also built today, making me think this is a library update issue somewhere) and I made the above simple environment to try to narrow things down, but the issue remains.

## Environment

Linux Cinnamon using Anaconda with Firefox, the OS is all up to date.

```
$ inxi -b
System: Host: ian-XPS-15-9550 Kernel: 5.4.0-77-generic x86_64 bits: 64 Desktop: Cinnamon 4.8.6
Distro: Linux Mint 20.1 Ulyssa
Machine: Type: Laptop System: Dell product: XPS 15 9550 v: N/A serial:
Mobo: Dell model: 0N7TVV v: A00 serial: UEFI [Legacy]: Dell v: 1.9.0 date: 10/11/2018
```

> originally posted by @ianozsvald on https://github.com/jupyterlab/jupyterlab/issues/10996