Make `_Config` and `Config` work as native class

In https://github.com/PyCQA/isort/pull/2586 we opt out of making _Config and Config compiled by mypyc as that caused issues.

Let's investigate why it doesn't work and make sure it does. This might require an upstream patch in mypy to fix the underlying bug. When we do this we can also remove the mypy-extensions dependency.