added py7zr
# What does this PR do?

This PR adds `py7zr` to use `samsum` as a dataset. 

```python
[1,14]<stdout>:    use_auth_token=use_auth_token,
[1,14]<stdout>:  File "/opt/conda/lib/python3.6/site-packages/datasets/load.py", line 448, in prepare_module
[1,14]<stdout>:    f"To be able to use this {module_type}, you need to install the following dependencies"
[1,14]<stdout>:ImportError: To be able to use this dataset, you need to install the following dependencies['py7zr'] using 'pip install py7zr' for instance'
```
Thanks a lot for adding this!