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

hmm, the only issue is this doesn't quite work for duplicated names, but i guess this is friendlier for readers of the error message. cc @TomAugspurger @jorisvandenbossche  Yeah duplicates labels was my initial concern.

Can we update the error message to keep using `.iloc[:, i]`, but also include `column name=" "` somewhere? Thanks for the feedback!
I see, I didn't think about duplicate names (and didn't know that they are allowed at all). Hello @moi90! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-01-06 20:45:56 UTC Sorry for the multiple commits. Feel free to squash. @moi90 no need to worry about the squashing, but we do need to get the tests passing.  looks like there are a handful of places where you need to update the expected error message I will look into this. @moi90 any luck? If you have questions feel free to post them on there :) @moi90 if you can use f-strings and ping on green. @moi90 can you rebase the CI error in the linter usually means there is an unused import somehwere No, it's an error in flake8:

```
   File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.7/site-packages/flake8/formatting/default.py", line 39, in format
    "col": error.column_number,
ValueError: unsupported format character ':' (0x3a) at index 41
```

I'm beginning to give up... Can't someone of the core team bring it to a successful conclusion? ```
ValueError: unsupported format character ':' (0x3a) at index 41
```

The last few times I've seen this, it turned out there was an unused import somewhere. can you merge master @moi90 I merged master and fixed some flake8 issues. Not sure why it failed before, but know that you can always run the linting checks locally (I recommend installing git commit hooks for this: https://dev.pandas.io/docs/development/contributing.html#python-pep8-black).

All green now, so will merge. Thanks for the PR! Wonderful, thanks! And you're welcome :)