ENH: Show column name in assert_frame_equal
The output of the AssertionError raised by assert_frame_equal is changed:

- old: `AssertionError: Attributes of DataFrame.iloc[:, 1] are different`
- new: `AssertionError: Attributes of DataFrame.loc[:, 'b'] are different`

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