Fixed error import Digraph

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[haeter525]

Hi @nplesak. Thank you for the contribution!
For better management, I would like to put a link to the related issue here. #286

[codecov-commenter]

Codecov Report

Merging #288 (a3e404b) into master (17539fe) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #288   +/-   ##
=======================================
  Coverage   80.14%   80.14%           
=======================================
  Files          49       49           
  Lines        3576     3576           
=======================================
  Hits         2866     2866           
  Misses        710      710           

Flag	Coverage Δ
unittests	80.14% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
quark/utils/output.py	98.63% <100.00%> (ø)

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[haeter525]

@nplesak LGTM, Thank you for this patch! 👍️