Remove dependency on pytest for running tests
Author: aaugustin
Pull request: yes


Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2055?src=pr&el=h1) Report
> Merging [#2055](https://codecov.io/gh/huggingface/transformers/pull/2055?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/35ff345fc9df9e777b27903f11fa213e4052595b?src=pr&el=desc) will **decrease** coverage by `0.45%`.
> The diff coverage is `95.45%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2055/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2055?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #2055      +/-   ##
=========================================
- Coverage   83.16%   82.7%   -0.46%     
=========================================
  Files         109     109              
  Lines       15858   15943      +85     
=========================================
- Hits        13188   13186       -2     
- Misses       2670    2757      +87
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2055?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX29wZW5haS5weQ==) | `81.45% <0%> (-0.55%)` | :arrow_down: |
| [transformers/tokenization\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rva2VuaXphdGlvbl94bG5ldC5weQ==) | `90.24% <100%> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_auto\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2F1dG9fdGVzdC5weQ==) | `36.36% <100%> (ø)` | :arrow_up: |
| [transformers/tokenization\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rva2VuaXphdGlvbl9hbGJlcnQucHk=) | `89.74% <100%> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_openai\_gpt\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX29wZW5haV9ncHRfdGVzdC5weQ==) | `94.73% <100%> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_bert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2JlcnRfdGVzdC5weQ==) | `96.22% <100%> (ø)` | :arrow_up: |
| [transformers/tests/tokenization\_auto\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl9hdXRvX3Rlc3QucHk=) | `50% <100%> (ø)` | :arrow_up: |
| [transformers/tests/tokenization\_distilbert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl9kaXN0aWxiZXJ0X3Rlc3QucHk=) | `63.63% <100%> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_roberta\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX3JvYmVydGFfdGVzdC5weQ==) | `75.2% <100%> (ø)` | :arrow_up: |
| [transformers/tests/tokenization\_transfo\_xl\_test.py](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl90cmFuc2ZvX3hsX3Rlc3QucHk=) | `97.43% <100%> (ø)` | :arrow_up: |
| ... and [39 more](https://codecov.io/gh/huggingface/transformers/pull/2055/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2055?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2055?src=pr&el=footer). Last update [35ff345...61978c1](https://codecov.io/gh/huggingface/transformers/pull/2055?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Given that the PR touches the whole test suite and that tests pass, if there's no opposition,I'd like to merge it before master diverges.

We can figure out running tests on the GPU on CircleCI separately.

Agreed. Squashed and merged.