BUG: Support timespec argument in Timestamp.isoformat() (#26131)
- [x] closes #26131
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I have just tried to support the same behaviour as `datetime.datetime()`, without adding the additional `"nanoseconds"` option, but this could be handled as well.
This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Thanks for the PR, but going to close due the lack of activity. Let us know if you'd like to continue working on this.  I'm planning to take over this PR as the original author seems to have lost time/interest.