Deprecation warning due to invalid escape sequences in Python 3.7
# 🐛 Bug

## To reproduce

Deprecation warnings are raised due to invalid escape sequences. This can be fixed by using raw strings or escaping the literals.

Steps to reproduce the behavior:

```
 find . -iname '*.py' | grep -v example | xargs -P 4 -I{} python3.8 -Wall -m py_compile {} 
./src/transformers/tokenization_transfo_xl.py:123: DeprecationWarning: invalid escape sequence \:
  self.punctuation_symbols = '!"#$%&()*+,-./\:;<=>?@[\\]^_`{|}~'  # noqa: W605
./src/transformers/tokenization_transfo_xl.py:150: DeprecationWarning: invalid escape sequence \s
  look_ahead_to_match_all_except_space = "(?=[^\s])"  # noqa: W605
```

## Expected behavior

No warnings

## Environment info
     
- `transformers` version: master branch
- Platform: linux
- Python version: 3.7
- PyTorch version (GPU?):
- Tensorflow version (GPU?):
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

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.
<|||||>The bug is still valid and I have raised https://github.com/huggingface/transformers/pull/4924