ENH: categorical scatter plot
- [ ] closes #12380 (this idea was mentioned as a comment here). Also, closes #31357 (I hadn't noticed found that issue when I first opened this)
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Here's what this would look like:

![image](https://user-images.githubusercontent.com/33491632/82671464-cd9ce500-9c36-11ea-9f1d-eb901d209ab9.png)


Was just hoping to get initial feedback on whether this would be welcome

Somethings I still need to do:
- [ ] add docstring / annotations to `_plot_colorbar`
- [x] `test_fash.sh` passes locally but there are some CI failures, need to reproduce / figure out why (those tests pass now, failure was probably unrelated)
- [ ] add an example of this functionality to the docs
- [ ] if ints are passed, treat same way as categorical