Using the recent ipykernel 7.x causes plots to not show up from ipympl widgets.

After upgrading to recent versions of `jupyterlab`, `ipympl` and `ipykernel` I observed on different systems, either Windows 11 or Linux, that `ipympl` figures would not show up sometimes without an explicit `plt.show()` at the end of the cell.

While this looked to me at first glance like an `ipympl` bug because it does not appear using inline figures it turns out that downgrading `ipykernel` to `6.3.1` completely solves the issue.

Here is the the original report with more details: https://github.com/matplotlib/ipympl/issues/609

I saw that `ipykernel>=7` has quite a few changes regarding `matplotlib`, so I assume this is a unwanted side-effect of these changes.