Allow pathlib.Path to be used on save_pretrained and save_vocabulary
Related to #4541

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4650?src=pr&el=h1) Report
> Merging [#4650](https://codecov.io/gh/huggingface/transformers/pull/4650?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e444648a302dc8520beec96356a4bf500944355c&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `71.42%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4650/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4650?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4650   +/-   ##
=======================================
  Coverage   77.42%   77.43%           
=======================================
  Files         128      128           
  Lines       21017    21019    +2     
=======================================
+ Hits        16273    16276    +3     
+ Misses       4744     4743    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4650?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4650/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.54% <71.42%> (+0.02%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4650/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4650?src=pr&el=footer). Last update [e444648...d453872](https://codecov.io/gh/huggingface/transformers/pull/4650?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>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.
