BUG: asymmetric error bars for series (GH9536)
closes #9536

This fix enables asymmetric errors bars for pd.Series plots. In the current implementation the bars are symmetric, even if two sets of errors are provided.

Took inspirations from #12046
Hello @Steffen911! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-07-17 13:40:41 UTC @MarcoGorelli @datapythonista Could one of you review the PR? Or maybe suggest another reviewer?  lgtm can u merge master and ping on green lgtm ping on green @jreback All tests are green. Travis reports itself as "Passed" when I go to the "Details", but apparently it didn't update the PR check after it has finished. Thanks @Steffen911!