DEPS: broken openpyxl >= 2.4.5 builds (on 3.6)
https://bitbucket.org/openpyxl/openpyxl/issues/790/conda-forge-builds-of-openpyxl-245-are

worked around for now:
https://github.com/pandas-dev/pandas/commit/d1e1ba08ef259724ba71e0953c52e8e4ad81bd17
