Allow pathlib PoxisPath in Dataset.read_json

**Is your feature request related to a problem? Please describe.**
```
from pathlib import Path
from datasets import Dataset
ds = Dataset.read_json(Path('data.json'))
```
causes an error
```
AttributeError: 'PosixPath' object has no attribute 'decode'
```

**Describe the solution you'd like**

It should be able to accept PosixPath and read the json from inside.