Nerf the old "isodatetime" package on PyPI

[MetRonnie]

We ought to make isodatetime a dead-end package which tells users to use metomi-isodatetime instead

[cpelley]

Why not the other way around?
Update isodatetime to be the definitive package??

I tried pip installing isodatetime by accident through an action only to then discover I should have been using metomi-isodatetime