[style] add pandas to setup.cfg
Need to add pandas to setup.cfg. Otherwise, for people that have pandas installed locally isort tries to change `eli5_utils.py` everytime.

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5093?src=pr&el=h1) Report
> Merging [#5093](https://codecov.io/gh/huggingface/transformers/pull/5093?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/90c833870c78bb3d5d807a9a3e6a40d24bf2302b&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5093/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5093?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5093   +/-   ##
=======================================
  Coverage   77.28%   77.28%           
=======================================
  Files         133      133           
  Lines       22134    22134           
=======================================
  Hits        17107    17107           
  Misses       5027     5027           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5093?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5093/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.26% <0.00%> (-0.16%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5093/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `75.40% <0.00%> (+0.40%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5093?src=pr&el=footer). Last update [90c8338...eae4841](https://codecov.io/gh/huggingface/transformers/pull/5093?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
