[FIX] Readwrite: use unicode

[jerneju]

Issue

Similar to biolab/orange3#2416

Description of changes

Includes

• Code changes
• Tests
• Documentation

[jerneju]

https://sentry.io/biolab/orange3-network/issues/434545244/

[codecov-io]

Codecov Report

Merging #70 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #70   +/-   ##
=======================================
  Coverage   44.18%   44.18%           
=======================================
  Files          17       17           
  Lines        2234     2234           
=======================================
  Hits          987      987           
  Misses       1247     1247

Impacted Files	Coverage Δ
orangecontrib/network/readwrite.py	20.18% <0%> (ø)	⬆️

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