DEP: update Versioneer
bump versioneer from 0.15 to 0.19

```
(pandas-dev) fangchenli@Fangchens-MacBook-Pro pandas-fangchenli % python setup.py version
running version
keywords are unexpanded, not using
got version from VCS {'version': '1.2.0.dev0+1185.g82894a4b6', 'full-revisionid': '82894a4b6329b8af74bb9c593858453a01b91bf6', 'dirty': False, 'error': None, 'date': '2020-11-14T16:24:02-0600'}
Version: 1.2.0.dev0+1185.g82894a4b6
 full-revisionid: 82894a4b6329b8af74bb9c593858453a01b91bf6
 dirty: False
 date: 2020-11-14T16:24:02-0600
```
i suppose this doesn't really change much. thanks @fangchenli 