BUG: to_clipboard fails to format output for Excel
`DataFrame.to_clipboard` has been broken for pasting to excel.  Tables are copied with spaces as delimiters instead of tabs (#21104).  

This issue originated in https://github.com/pandas-dev/pandas/commit/e1d5a2738235fec22f3cfad4814e09e3e3786f8c#diff-3f25860d9237143c1952a1f93c3aae18R102
which I've partially reverted.  

By setting the delimiter to `r'\t'`, a 2 character string, obj.to_csv raised an error, but is was caught and passed silently.  I reverted the separator to `'\t'`. 

Similar issue in `read_clipboard` also fixed.


- [x] closes #21104