Deprecation warnings on python 3.8.x

Using python 3.8.5 on Linux, I get the following deprecation warnings:
```
python3.8/site-packages/jsonpath_rw_ext/_string.py:19: DeprecationWarning: invalid escape sequence \(
python3.8/site-packages/jsonpath_rw_ext/_string.py:20: DeprecationWarning: invalid escape sequence \(
```
Would it be possible to update code accordingly please ?
Thx a lot !
