BUG to_clipboard passes the wrong sep to to_csv
Originally, the sep is `\t` as to_csv expects. A commit changes the sep to `r'\t',` causing an exception (because `csv.writer` expects the delimiter to be a one-character string). The exception is suppressed (with `pass` in the `except` block), causing the data frame to be passed to `to_string` downstream (even though it should be handled by `to_csv`)

Changing `r't'` back to `\t` is a quick fix, but just `pass` in the `except` block seems like the more problematic root cause (that I'm not handling here). 

- [x ] closes #21385
- [ ] tests added / passed
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x ] whatsnew entry
