Fixes for more recent dask versions

[joezuntz]

Also a fix for describe_configuration run on incomplete classes.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (1e20f29) 97.61% compared to head (009ba8d) 89.81%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
- Coverage   97.61%   89.81%   -7.80%     
==========================================
  Files          15       16       +1     
  Lines        1676     1709      +33     
==========================================
- Hits         1636     1535     -101     
- Misses         40      174     +134     

Files	Coverage Δ
ceci/stage.py	90.68% <100.00%> (-8.91%)	⬇️
ceci/tools/ancestors.py	90.62% <90.62%> (ø)

... and 1 file with indirect coverage changes

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