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:

pydata/xarray#3961

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

pydata/xarray#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)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

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

Further checks

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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[dalonsoa]

If it works, it looks good!

[jacobcook1995]

LGTM!