Deprecate using `xlrd` engine in favor of openpyxl
xlrd is unmaintained and the previous maintainer has asked us to move towards openpyxl. xlrd works now, but *might* have some issues when Python 3.9 or later gets released and changes some elements of the XML parser, as default usage right now throws a `PendingDeprecationWarning`

Considering that I think we need to deprecate using `xlrd` in favor of `openpyxl`. We might not necessarily need to remove the former and it does offer some functionality the latter doesn't (namely reading `.xls` files) but should at the very least start moving towards the latter