Update codeparrot data preprocessing
This PR updates the preprocessing script of CodeParrot data (python files), we add new filters for:

- config and test files
- uncommon files (those without a mention of Python classic keyworks: `def`, `for`..) 
- unusual files (don't use the assignement `=` operator often)
- files with low ratio between number of charcaters and number of tokens after tokenization

The impact of some of these filters is analyzed in this [tweet](https://twitter.com/LoubnaBenAllal1/status/1514300881419878403?s=20&t=IkodNO5Ma3X866-Yj-LvQw).

cc @lvwerra
_The documentation is not available anymore as the PR was closed or merged._