Check if we still need `_werkzeug.py`

[antonpirker]

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?)