Fix RTD builds

[davidorme]

Description

This fixes the readthedocs.yml file to add the new approved RTD build for poetry project. Fixes #397. Also updates xarray. Fixes #396

This is mostly just to get the website building again so I can migrate the name.

Type of change

• [ ] New feature (non-breaking change which adds functionality)
• [ ] Optimization (back-end change that speeds up the code)
• [x] Bug fix (non-breaking change which fixes an issue)

Key checklist

• [x] Make sure you've run the pre-commit checks: $ pre-commit run -a
• [x] All tests pass: $ poetry run pytest

Further checks

• [x] Code is commented, particularly in hard-to-understand areas
• [x] Tests added that prove fix is effective or that feature works

[vgro]

To fast for me ;-)

[davidorme]

Sorry @vgro - I'm not sure of the etiquette when we want a super quick review of a small priority PR. Adding a ton of people and 'fastest fingers first' seems kinda spammy.