Fixes #457 - Bumped openpyxl dependency to 2.6.0

[codecov[bot]]

Codecov Report

Merging #458 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #458   +/-   ##
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  Coverage   90.42%   90.42%           
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  Files          28       28           
  Lines        2589     2589           
=======================================
  Hits         2341     2341           
  Misses        248      248

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[hugovk]

Thanks!