`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

Start by reproducing the issue in the minimal conda environment described, comparing breakpoint() with import pdb; pdb.set_trace() in a notebook cell. Then trace how ipykernel handles these debugger entry points. Done means breakpoint() invokes pdb in the reported JupyterLab environment, with the existing behavior covered against regression.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.