Update repo to isort v5
Author: sgugger
Pull request: yes

Since isort now works properly with black, we can use the latest version. It also comes with new functionality (hence the lot of diff) mainly:
- it can deal with the __init__
- it can deal with import in if/else blocks.

This will fix #6681

Also v5 does not use the recursive flag anymore, so I removed it from the make style and make quality commands. For users having an old isort version, this will result in make style/make quality having no impact.

It's very likely that users having suggested a PR will need to rebase after this is merged, update isort then run a new `make style`. We can also assist by force-pushing on their branches.

Commented on the changes I made manually, all the others come from the new version of isort.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6686?src=pr&el=h1) Report
> Merging [#6686](https://codecov.io/gh/huggingface/transformers/pull/6686?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1a779ad7ecb9e5215b6bd1cfa0153469d37e4274?el=desc) will **decrease** coverage by `0.49%`.
> The diff coverage is `92.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6686/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6686?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6686      +/-   ##
==========================================
- Coverage   79.65%   79.16%   -0.50%     
==========================================
  Files         156      156              
  Lines       28250    28254       +4     
==========================================
- Hits        22503    22366     -137     
- Misses       5747     5888     +141     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6686?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/commands/serving.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy9zZXJ2aW5nLnB5) | `0.00% <0.00%> (ø)` | |
| [src/transformers/commands/user.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy91c2VyLnB5) | `0.00% <ø> (ø)` | |
| [src/transformers/data/test\_generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Rlc3RfZ2VuZXJhdGlvbl91dGlscy5weQ==) | `0.00% <0.00%> (ø)` | |
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `82.13% <ø> (ø)` | |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `86.68% <ø> (ø)` | |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `89.45% <ø> (ø)` | |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.25% <ø> (ø)` | |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `71.61% <ø> (-12.22%)` | :arrow_down: |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `79.94% <0.00%> (ø)` | |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `42.48% <ø> (ø)` | |
| ... and [21 more](https://codecov.io/gh/huggingface/transformers/pull/6686/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6686?src=pr&el=footer). Last update [1a779ad...dbaad9c](https://codecov.io/gh/huggingface/transformers/pull/6686?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).