Bug#1115136: xarray: tests fail with mypy 1.18

[Rebecca N. Palmer]

Control: retitle -1 xarray: tests fail with mypy 1.18

This appears to be caused by mypy 1.18 changing this error message; 
according to xarray upstream, pytest-mypy-plugins can't parse the new 
format so they're skipping these tests for now.

https://github.com/pydata/xarray/commit/8c5706de02ad38a8c93f6c385ed7b83a18f3720d#diff-cbd902d45b1bd0cccf60b93331c1fe1360ea2227991866b8722d3a4cf5b3dc34