Faster bert basic tokenizer
In this PR I tried 2 things:

* First I change some comparisons in the form of `a == sth or a == otherthing` for `a in {sth, otherthing}`. It is faster and more readable.

* I noticed that some methods could actually be functions because they did not use anything from the class.

I am a bit new to this, so I will accept any feedback.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4721?src=pr&el=h1) Report
> Merging [#4721](https://codecov.io/gh/huggingface/transformers/pull/4721?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/47a551d17b6ed2eaf03301f049006d559fca5cf3&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/4721/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4721?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4721   +/-   ##
=======================================
  Coverage   77.14%   77.14%           
=======================================
  Files         128      128           
  Lines       21073    21072    -1     
=======================================
  Hits        16256    16256           
+ Misses       4817     4816    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4721?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/4721/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydC5weQ==) | `94.97% <100.00%> (-0.03%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4721/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.83% <0.00%> (+0.16%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4721?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/4721?src=pr&el=footer). Last update [47a551d...6f7ff85](https://codecov.io/gh/huggingface/transformers/pull/4721?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Rather than making them regular functions I'd make them into static methods since they're still very much related to the class. You just don't need `self`.<|||||>> Rather than making them regular functions I'd make them into static methods since they're still very much related to the class. You just don't need `self`.

Done<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
