Turning on single-threaded handling in dask

[davidorme]

Description

This PR is to fix segfaults in testing and docs building when xarray.open_mfdataset is used. This is due to underlying issues with the netCDF code not being thread safe. We previously turned on lock=False in that command, following advice here:

https://github.com/pydata/xarray/issues/3961

And that worked, but the issue has started occurring again. Following the advice in this issue thread:

https://github.com/pydata/xarray/issues/7079

this PR now explicitly sets the dask scheduler to use single-threading.

Fixes #354

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.75%. Comparing base (327ad85) to head (5ff2268).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #457 +/- ## ======================================== Coverage 94.75% 94.75% ======================================== Files 70 70 Lines 3791 3793 +2 ======================================== + Hits 3592 3594 +2 Misses 199 199 ```

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