Fix SS06 formatting errors
Errors resolved (from list in #29254):
pandas.DataFrame.filter
pandas.DataFrame.first
pandas.DataFrame.idxmax
pandas.DataFrame.idxmin
pandas.DataFrame.last
pandas.DataFrame.reindex
pandas.DataFrame.pivot_table
pandas.DataFrame.reorder_levels
pandas.DataFrame.unstack
pandas.DataFrame.melt

Validated with `python scripts/validate_docstrings.py`

Referencing issue: #27977

Hello @mcalingo! 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 2019-10-29 09:34:47 UTC Looks like `filter` has some lint complaints Thanks @mcalingo 