Added support for pathlib.Path objects instead of string paths in from_pretrained() (resolves #3962)
Resolves issue/feature request #3962 

This only covers string paths in `from_pretrained` (so vocab_file paths for` __init__` for tokenizers aren't covered). This may or may not be a feature that is desired (since it's fairly easy for the client to convert a path-like to a string). With the changes, you would be able to use `from_pretrained` with path-likes:

```
from pathlib import Path
from transformers import AutoModel, AutoTokenizer

my_path = Path("path/to/model_dir/")
AutoTokenizer.from_pretrained(my_path)
AutoModel.from_pretrained(my_path)
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3975?src=pr&el=h1) Report
> Merging [#3975](https://codecov.io/gh/huggingface/transformers/pull/3975?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/4e817ff41885063e08bb3bcd63e5adfd835b9911&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `85.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3975/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3975?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3975   +/-   ##
=======================================
  Coverage   78.44%   78.45%           
=======================================
  Files         111      111           
  Lines       18518    18534   +16     
=======================================
+ Hits        14527    14541   +14     
- Misses       3991     3993    +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3975?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/3975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `40.57% <50.00%> (-0.10%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `68.68% <75.00%> (-0.19%)` | :arrow_down: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.57% <100.00%> (+0.02%)` | :arrow_up: |
| [src/transformers/modelcard.py](https://codecov.io/gh/huggingface/transformers/pull/3975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGNhcmQucHk=) | `87.80% <100.00%> (+0.15%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.10% <100.00%> (+0.34%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.95% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/3975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `97.50% <100.00%> (+0.13%)` | :arrow_up: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.73% <100.00%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3975?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/3975?src=pr&el=footer). Last update [4e817ff...033d5f8](https://codecov.io/gh/huggingface/transformers/pull/3975?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>On second thought, I probably would't merge this. I think it's better to just let the user deal with this on their end.<|||||>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.
