revert openpyxl test changes
follow up to #30009 - thanks @jbrockmendel for fixing it and sorry I wasn't able to review more. I *think* the root cause though is just defusedxml being installed alongside things, which is only prevalent in our 3.6 build.

pinning openpyxl version alone I think is fine, but let's see...note that this same defusedxml issue causes a lot of warnings to be captured for xlrd in the excel tests