TST: Explicitly test with cores set to None

[scottclowe]

No description provided.

[codecov]

Codecov Report

Merging #222 (17fb37b) into master (0785e42) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #222   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files           8        8           
  Lines         916      916           
  Branches      184      184           
=======================================
  Hits          854      854           
  Misses         32       32           
  Partials       30       30           

Flag	Coverage Δ
nbsmoke	66.70% <ø> (ø)
unittests	92.79% <ø> (ø)

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