Update tokenizers to 0.7.0-rc5
Author: n1t0
Pull request: yes

This includes some bug fix (around added tokens), and a small breaking change.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3705?src=pr&el=h1) Report
> Merging [#3705](https://codecov.io/gh/huggingface/transformers/pull/3705?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/bc65afc4dfac3badf3de3be395d4023b44c61bdd&el=desc) will **decrease** coverage by `0.11%`.
> The diff coverage is `50.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3705/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3705?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3705      +/-   ##
==========================================
- Coverage   78.14%   78.02%   -0.12%     
==========================================
  Files         104      104              
  Lines       17723    17710      -13     
==========================================
- Hits        13849    13818      -31     
- Misses       3874     3892      +18     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3705?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `82.50% <0.00%> (ø)` | |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `40.67% <100.00%> (-0.29%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `85.76% <0.00%> (-1.77%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `21.32% <0.00%> (-0.58%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `96.07% <0.00%> (-0.47%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.96% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.84% <0.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `85.53% <0.00%> (-0.05%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `90.40% <0.00%> (-0.03%)` | :arrow_down: |
| ... and [7 more](https://codecov.io/gh/huggingface/transformers/pull/3705/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3705?src=pr&el=footer). Last update [bc65afc...e31554a](https://codecov.io/gh/huggingface/transformers/pull/3705?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).