Check if we still need `_werkzeug.py`

The file `_werkzeug.py` contains some vendored in code from very old versions of werkzeug. 

Please check:
- Is this code used anywhere (I guess so)
- Can we update the code to newer code from Werkzeug?

Also make sure this code is tested to have a high test coverage. (maybe by vendoring in the related tests from Werkzeug?)