Add packaging to setup.py
Running `pip install -e transformers` and then `python -c "import transformers"` fails on a fresh Docker container with the error:

```bash
ModuleNotFoundError: No module named 'packaging'
Thu May 21 21:59:44 2020<stderr>:Traceback (most recent call last):
Thu May 21 21:59:44 2020<stderr>:  File "/.../", line 37, in <module>
Thu May 21 21:59:44 2020<stderr>:    from transformers import (
Thu May 21 21:59:44 2020<stderr>:  File "/fsx/transformers/src/transformers/__init__.py", line 350, in <module>
Thu May 21 21:59:44 2020<stderr>:    from .trainer import Trainer, set_seed, torch_distributed_zero_first, EvalPrediction
Thu May 21 21:59:44 2020<stderr>:  File "/fsx/transformers/src/transformers/trainer.py", line 14, in <module>
Thu May 21 21:59:44 2020<stderr>:    from packaging import version
```

Looks like this dependency was recently added, so adding it to setup.py requirements.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4509?src=pr&el=h1) Report
> Merging [#4509](https://codecov.io/gh/huggingface/transformers/pull/4509?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/865d4d595eefc8cc9cee58fec9179bd182be0e2e&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4509/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4509?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4509      +/-   ##
==========================================
- Coverage   77.90%   77.88%   -0.02%     
==========================================
  Files         123      123              
  Lines       20472    20472              
==========================================
- Hits        15949    15945       -4     
- Misses       4523     4527       +4     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4509?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/hf\_api.py](https://codecov.io/gh/huggingface/transformers/pull/4509/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9oZl9hcGkucHk=) | `93.06% <0.00%> (-4.96%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4509/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4509/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.53% <0.00%> (+0.11%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4509/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (+0.16%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4509?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4509?src=pr&el=footer). Last update [865d4d5...ffd7187](https://codecov.io/gh/huggingface/transformers/pull/4509?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This dependency was recently added, but it was not intended. It was removed with https://github.com/huggingface/transformers/commit/10d72390c029b3f139639621fb9a3a264560e05b. Thanks for offering a fix!