ENH: Informative dtype message for for assert_series_equal
Pass obj to assert_attr_equal in assert_series_equal.

- [ ] closes #28991
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
- [x] Fix example in `assert_frame_equal` docstring
Which whatsnew do I have to change? If there's nothing user-facing, you dont need a whatsnew entry OK thanks! Then this pull request should be ready to merge. can you merge master. ping when all green. @moi90 can you rebase?  the CI failures should be fixed now thanks @moi90  You're welcome!