Fix importing transformers on Windows - SIGKILL not defined
On Windows platform there is no such `signal.SIGKILL`, we need to send `CTRL_C_EVENT` which is `Ctrl+C`. 

This PR aims at: 

1. Not importing the `signal.SIGKILL` on Windows platforms as it's undefined
2. Using the right `signal.CTRL_C_EVENT` on Windows platforms and `signal.SIGKILL` on Unix*.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4997?src=pr&el=h1) Report
> Merging [#4997](https://codecov.io/gh/huggingface/transformers/pull/4997?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9931f817b75ecb2c8bb08b6e9d4cbec4b0933935&el=desc) will **increase** coverage by `0.72%`.
> The diff coverage is `66.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4997/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4997?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4997      +/-   ##
==========================================
+ Coverage   76.89%   77.61%   +0.72%     
==========================================
  Files         128      128              
  Lines       21854    21856       +2     
==========================================
+ Hits        16804    16964     +160     
+ Misses       5050     4892     -158     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4997?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/benchmark/benchmark\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4997/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX3V0aWxzLnB5) | `72.96% <66.66%> (-0.14%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4997/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.71% <0.00%> (-1.56%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/4997/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `75.85% <0.00%> (+19.75%)` | :arrow_up: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4997/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `56.68% <0.00%> (+28.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4997?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/4997?src=pr&el=footer). Last update [9931f81...d6f30c5](https://codecov.io/gh/huggingface/transformers/pull/4997?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great, thanks @mfuntowicz 