Pandas 1.3 renamed ABCIndexClass to ABCIndex

With Pandas 1.3.0 soft-releasing today, checks for Python 3.7 are failing to import ```pandas.core.dtypes.generic.ABCIndexClass``` as it was renamed to ```ABCIndex```.

The Python 3.7 checks are the only ones currently affected by this issue, since it's the only version of Python with a Pandas 1.3.0 release.