CI: troubleshoot openpyxl failures
tested on OSX 36 and 37, we'll see if that generalizes
@jorisvandenbossche I think noticed this last week > @jorisvandenbossche I think noticed this last week

Right [here](https://github.com/pandas-dev/pandas/pull/29862#issuecomment-559013181) (comment). is there a better workaround someone has in mind to get the CI back to green? @TomAugspurger gentle ping thanks @jbrockmendel happy to keep Ci green
keep an eye open for when can revert Might be awhile according to openpyxl devs:

https://bitbucket.org/openpyxl/openpyxl/issues/1373/broken-writer-with-lxml-defusedxml