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

@anhqle : Thanks for this!  Going to need a test to protect against this in the future. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21398?src=pr&el=h1) Report
> Merging [#21398](https://codecov.io/gh/pandas-dev/pandas/pull/21398?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/415012f4f38ca0cf41717c51e49bd2349cba09a8?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21398/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21398?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21398   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files         153      153           
  Lines       49596    49596           
=======================================
  Hits        45576    45576           
  Misses       4020     4020
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <ø> (ø)` | :arrow_up: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21398?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/clipboards.py](https://codecov.io/gh/pandas-dev/pandas/pull/21398/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NsaXBib2FyZHMucHk=) | `100% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21398?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21398?src=pr&el=footer). Last update [415012f...9e65713](https://codecov.io/gh/pandas-dev/pandas/pull/21398?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Close because duplicate of #21111 