Legacy editable install

[jonmaddock]

When building, the final message I see after a successful install is:

      Successfully uninstalled process-3.1.0
  DEPRECATION: Legacy editable install of process[examples,test]==3.1.0 from file:///home/jon/code/process (setup.py develop) is deprecated. pip 25.0 will enforce this behaviour change. A possible replacement is to add a pyproject.toml or enable --use-pep517, and use setuptools >= 64. If the resulting installation is not behaving as expected, try using --config-settings editable_mode=compat. Please consult the setuptools documentation for more information. Discussion can be found at https://github.com/pypa/pip/issues/11457
  Running setup.py develop for process

We should probably move to a pyproject.toml or similar.

[clmould]

Will be closed by #3983