TST add test case for drop_duplicates
Add a test case to drop_duplicates for inplace=True.
Hello @r-toroxel! 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 2020-07-07 17:55:33 UTC No.
Its just expanding on test coverage, after looking at pytest-cov and existing tests. thanks @r-toroxel 

happy to take PRs to verify other inplace ops return None thanks. yes. created an issue. https://github.com/pandas-dev/pandas/issues/35179 