CI: Unpin openpyxl
Followup to https://github.com/pandas-dev/pandas/pull/22601 and https://github.com/dask/dask-ml/issues/84#issuecomment-418640761.

Upstream issue: https://bitbucket.org/openpyxl/openpyxl/issues/1094/error-with-pandas-datetimeindex

(one of several in fact filed for this issue)