DOC: replace ggpy with plotnine on the ecosystem page

[jorisvandenbossche]

We have an entry about ggpy in the visualization section of the ecosystem page (https://dev.pandas.io/docs/ecosystem.html#visualization). However, ggpy is no longer maintained (last commit is more than 3 years ago: https://github.com/yhat/ggpy). While plotnine is an actively developed alternative ggplot-python-interface (https://github.com/has2k1/plotnine/).

So I think we can just replace the entry about ggpy with one about plotnine.