Replaced value assignment by location accessor.
Changed self.obj[key] = value to self.obj.loc[:, key] = value to disable warning.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Thanks for the pr @kosmitive.  Is there an issue associated with this? What is the behavior change here (and if there is, it should be tested). Also seems like a change that might be performance sensitive
 I get a warning otherwise that the old command operates on a copy, so I thought changing it to in-place assignment would make sense and be a really quick fix. But I didn't do further investigation on that line of code.  @mzeitlin11 @jreback  Here is the warning I get with python 3.9

.../.venv/lib/python3.9/site-packages/pandas/core/indexing.py:1597: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead @kosmitive pls file an issue with a reproducible test. The code you are touching is not trivially changed and has quite some implications. This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Thanks for the PR, but agreed it would be better if an issue could be raised first so the entirety of the issue could be understood before diving into a fix. Closing.