Transformer-XL: Improved tokenization with sacremoses
Fixes #5136

As explained in the above issue, this PR fixes the tokenization of the TransfoXLTokenizer by using the sacremoses library with an extended feature of tokenizing comma-separated and floating point numbers. That way the input text is tokenized the same way as in the WikiText-103 dateset used for pretraining.

Changes in a nutshell:
 * The TransfoXLTokenizer is now using sacremoses for tokenization
 * Added tokenization of comma-separated and floating point numbers.
 * Removed prepare_for_tokenization() from tokenization_transfo_xl.py because punctuation is handled by sacremoses
 * Added corresponding tests
 * Removed test comapring TransfoXLTokenizer and TransfoXLTokenizerFast (as discussed in #5302)
 * Added deprecation warning to TransfoXLTokenizerFast (as discussed in #5302)

@TevenLeScao 
`ci/circleci: check_code_quality` fails for me due to Python 3.6 is not compatible with PyTorch 1.6. Any ideas how to fix this?<|||||>Pinging @n1t0 and @TevenLeScao (in holidays right now, will be back next week!)<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6322?src=pr&el=h1) Report
> Merging [#6322](https://codecov.io/gh/huggingface/transformers/pull/6322?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/930153e7d2d658267b7630a047a4bfc85b86042d?el=desc) will **increase** coverage by `0.37%`.
> The diff coverage is `96.96%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6322/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6322?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6322      +/-   ##
==========================================
+ Coverage   79.36%   79.74%   +0.37%     
==========================================
  Files         157      157              
  Lines       28569    28587      +18     
==========================================
+ Hits        22675    22797     +122     
+ Misses       5894     5790     -104     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6322?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `41.74% <96.96%> (-0.75%)` | :arrow_down: |
| [src/transformers/commands/env.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy9lbnYucHk=) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [src/transformers/commands/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy9fX2luaXRfXy5weQ==) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [src/transformers/commands/transformers\_cli.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy90cmFuc2Zvcm1lcnNfY2xpLnB5) | `0.00% <0.00%> (-87.50%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `18.94% <0.00%> (-74.32%)` | :arrow_down: |
| [src/transformers/commands/download.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy9kb3dubG9hZC5weQ==) | `0.00% <0.00%> (-65.00%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.53% <0.00%> (-63.81%)` | :arrow_down: |
| [src/transformers/commands/serving.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy9zZXJ2aW5nLnB5) | `0.00% <0.00%> (-55.89%)` | :arrow_down: |
| [src/transformers/commands/run.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy9ydW4ucHk=) | `0.00% <0.00%> (-53.34%)` | :arrow_down: |
| [src/transformers/commands/user.py](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy91c2VyLnB5) | `0.00% <0.00%> (-36.56%)` | :arrow_down: |
| ... and [22 more](https://codecov.io/gh/huggingface/transformers/pull/6322/diff?src=pr&el=tree-more) | |

------

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