Add a now required pandas dep for 2.6.

see this bug report: pandas-dev/pandas#7284

Author: Cadair

.travis.yml

@@ -49,7 +49,7 @@ install:
    - pip install $PIP_WHEEL_COMMAND numpy==$NUMPY_VERSION
    - pip install $PIP_WHEEL_COMMAND matplotlib
    - pip install $PIP_WHEEL_COMMAND scipy
-   - pip install $PIP_WHEEL_COMMAND pandas==$PANDAS_VERSION
+   - pip install $PIP_WHEEL_COMMAND openpyxl==1.8.6 pandas==$PANDAS_VERSION
    - pip install $PIP_WHEEL_COMMAND astropy>=0.3.1
    - pip install $PIP_WHEEL_COMMAND $SUDS_PKG beautifulsoup4 requests
    - pip install $PIP_WHEEL_COMMAND pytest