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