Use with_suffix to change the extension of the path
As per https://github.com/huggingface/transformers/pull/3934#discussion_r421307659. Note that this does not rename the file, it simply changes the path 'string' to another one with a different suffix.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4203?src=pr&el=h1) Report
> Merging [#4203](https://codecov.io/gh/huggingface/transformers/pull/4203?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/026097b9ee7862905ec3f3b0e729c5dbc95a0fd9&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4203/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4203?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4203      +/-   ##
==========================================
+ Coverage   78.43%   78.44%   +0.01%     
==========================================
  Files         120      120              
  Lines       19800    19797       -3     
==========================================
  Hits        15530    15530              
+ Misses       4270     4267       -3     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4203?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/hf\_argparser.py](https://codecov.io/gh/huggingface/transformers/pull/4203/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9oZl9hcmdwYXJzZXIucHk=) | `61.97% <0.00%> (+1.16%)` | :arrow_up: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4203/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `41.83% <0.00%> (-0.35%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4203/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.77% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4203/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `74.38% <0.00%> (+0.41%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4203?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/4203?src=pr&el=footer). Last update [026097b...a73d487](https://codecov.io/gh/huggingface/transformers/pull/4203?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks!