Fix: Warning(Attempt to set value to copy of a slice)
Getting a warning emitted from this line of code.  Here is the fix I used to get rid of the warning, and here is the code that generated it on my setup (Anaconda3, Windows 10):

```
import pandas as pd

ts_columns = []
for col in df.columns:
    if isinstance(df[col].dtype, Timestamp):
        ts_columns.append(col)
```

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
