Restrict dev environment to python<3.12

[atedstone]

Resolves #428 .

[rhugonnet]

@atedstone You also need to add the restriction in environment.yml, otherwise a specific test in CI fails! (the two files always need to be consistent for the main dependencies)

[atedstone]

@rhugonnet so it seems. I'll add to environment.yml as well but this does seem like a 'sledgehammer to crack a nut' approach!