ipykernel 7.0.0 release with subshells

# Kernel Subshells in ipykernel 7.0.0

This is a placeholder issue for the `ipykernel 7.0.0` release which includes kernel subshells for the first time in a full release, and is due for release on Monday 13th October 2025. This is intended to be the first point of call to obtain information about the release, ask questions and report problems with it.

For those not using subshells it is supposed to be backward compatible with the `6.x` branch, but there are some architectural changes such as the use of a separate thread to handle sending and receiving of shell channel messages which may break assumptions made in downstream libraries and hence this is identified as a major releases as it is potentially backwards incompatible. If you experience problems then please report them, and you can pin `ipykernel < 7` if necessary.

There have been many discussions, issues and pull requests about subshells over the last few years. Linking to all of these would be overwhelming, so here are a few salient links only. Searching in the `ipykernel` github repo for "subshells" will give more links.

The original motivation and changes to the Jupyter protocol are in [Jupyter Enhancement Proposal 91](https://github.com/jupyter/enhancement-proposals/pull/91).

The history in `ipykernel` is complicated by the original implementation being included with changes from `tornado` to `anyio`, but the `anyio` changes have since been sidelined and the second subshell implementation in the current `7.x` branch is on top of `tornado`. Version numbers in related issues and PRs are therefore not always correct.

## ipykernel

- Original implementation in PR #1249, this has a useful diagram and a list of future changes, such as to execution counts, that need to be made.
- Current implementation in PR #1396.
- Details of branch manipulations in #1387 and #1399.

## jupyterlab

- Subshell support added in jupyterlab/jupyterlab#16963. This includes a video and explanation of how you can use subshells in kernels that support them.
- Support for comms over subshells added in jupyterlab/jupyterlab#17363 and jupyterlab/jupyterlab#17364, including some useful videos. This fixes a problem in `ipywidgets` (and hence other widget libraries) of widgets not updating whilst the kernel shell is performing a blocking operation.

## ipympl

- `ipympl 0.9.8` includes matplotlib/ipympl#603 to support comms over subshells.

## Future work

There is future work to be done supporting the `ipykernel 7.0.0` release in downstream projects, and in addressing various issues that need to work with subshells such as those enumerated in the first post of #1249.