Add obj=obj to get it in failing assert messages
Adding obj=obj in this call to assert_attr_equal helps make failing test cases more easily debuggable. This follows the pattern elsewhere in this same file of other calls to assert_attr_equal and related methods.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

The Checks_and_doc errors seem to be for style violations in other files not touched in this PR: https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=9364 Apologies. I do see some test failures related to this change, e.g.:

AssertionError: Pattern 'Attributes are different' not found in 'DataFrame.iloc[:, 0] are different

Perhaps this example also suggests that the error messages will be more readable if they still mention "Attributes".

I will try to close this now and be more careful next time about running all the relevant tests.