fix parallel computation

1 job for master in 13 minutes and 48 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Test
failed build #638242

00:13:48

 
Name Stage Failure
failed
build Test
RuntimeError: 
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There was an error in building your book. Look above for the cause.

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Cleaning up file based variables
ERROR: Job failed: exit code 1