Document that Pandas 0.25.2 should be compatible with Python 3.8

[TomAugspurger]

#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 conda-forge/python-feedstock#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

[shafaypro]

GG mate, Yes Indeed.

[TomAugspurger]

Reran with Python 3.8b4. Things seem OK, but a bunch of plotting tests are failing.

[lizsaret]

@TomAugspurger can i work on this?

[TomAugspurger]

This is currently blocked by conda-forge/python-feedstock#271. I'm a bit hesitant to state that we're 3.8 compatible until we're actually testing on CI.

[jreback]

agreed we have no testing on 3.8 yet