Fix SyntaxWarning with python 3.8 due to using "is" in a literal comparison
Created originally on Bitbucket by aerostitch
Was already merged in Bitbucket before import, marked as merged by the import user
This PR fixes a SyntaxWarning you get when running openpyxl with python 3.8 due to using "is" in a literal comparison
The warning was:
SyntaxWarning: "is" with a literal. Did you mean "=="?