Add strip_accents to basic BertTokenizer.
The BertTokenizerFast can turn off strip_accents with `strip_accents=False`. This PR also adds this option to the basic BertTokenizer.

Also see #6186
Strange CI problem with checksum of Torch:
```
ERROR: THESE PACKAGES DO NOT MATCH THE HASHES FROM THE REQUIREMENTS FILE. If you have updated the package versions, please update the hashes. Otherwise, examine the package contents carefully; someone may have tampered with them.
    torch from https://files.pythonhosted.org/packages/38/53/914885a93a44b96c0dd1c36f36ff10afe341f091230aad68f7228d61db1e/torch-1.6.0-cp36-cp36m-manylinux1_x86_64.whl#sha256=7669f4d923b5758e28b521ea749c795ed67ff24b45ba20296bc8cff706d08df8 (from transformers==3.0.2):
        Expected sha256 7669f4d923b5758e28b521ea749c795ed67ff24b45ba20296bc8cff706d08df8
             Got        8188c461d0b762aa4ae6e72105b3c3a01f7bb2863b46a392ff8537a0854e8967
```<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6280?src=pr&el=h1) Report
> Merging [#6280](https://codecov.io/gh/huggingface/transformers/pull/6280?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/31da35cc8939e19a292cb7f15cad5c9a1ddf7f23&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6280/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6280?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #6280   +/-   ##
=======================================
  Coverage   79.64%   79.64%           
=======================================
  Files         147      147           
  Lines       27120    27125    +5     
=======================================
+ Hits        21600    21605    +5     
  Misses       5520     5520           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6280?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6280/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydC5weQ==) | `91.51% <100.00%> (+0.19%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6280?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/6280?src=pr&el=footer). Last update [31da35c...547df96](https://codecov.io/gh/huggingface/transformers/pull/6280?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>You can ignore the HASH errors, we're working on solving these but they're unrelated to your PR.<|||||>This is ready to be merged IMO. 😁<|||||>Wow - this was really fast from first commit to merge. Many thanks to the contributors. This makes open source development twice as much fun. 