Support tox v4 and add action to generate lockfile

At the moment we use tox to:
- Run specific tox envs in CI and locally, like `tox -e docs` to invoke `sphinx` with the right args including for `autobuild`
- Run `tox -p` to do the tasks that CI will run, but in parallel in a single terminal under a single version of python
- Do this without making a venv for each env with `tox-direct`

We considered using `nox`, but it doesn't have a parallel option at the moment as per https://github.com/wntrblm/nox/issues/544

We would like to add the ability to do something like `tox -e lock` to generate `dev-requirements.txt` as specified in https://github.com/DiamondLightSource/blueapi/pull/499

To do this we should should update to tox 4 and work out what changes are needed to support the current workflow, and also work out how to add a `lock` command that will not be run by the equivalent of `tox -p`