TST: Fix assertNotIsInstance msg
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

The current error message below is confusing.

```
tm.assertNotIsInstance(pd.Series([1]), pd.Series)
# AssertionError: Expected object to be of type <class 'pandas.core.series.Series'>, found <class 'pandas.core.series.Series'> instead
```
