Document that Pandas 0.25.2 should be compatible with Python 3.8
https://github.com/pandas-dev/pandas/pull/28101 gets things passing with Python 3.8 locally.

Ideally, we'd have 3.8 actually tested on CI before releasing 0.25.2. Hopefully once https://github.com/conda-forge/python-feedstock/pull/271 is up, we can do that (and maybe build our deps from source).

Once we're tested, we should add a note to the whatsnew saying that pandas 0.25.2 is compatible with Python 3.8