The context manager uses `NamedTemporaryFile` to store the current configuration, to later restore them.
Instead of passing the file object directly to the save function, it just passes the file name, i.e. the save (and the load function) will open the file again, which is in itself not a problem.
However, on the Github Windows image this leads to a permission error (using the created file object is fine).

This commit solves this by adding the `file` argument to `Config.save()` that allows to pass a file object directly to the function.
The same change is applied to the load function of the config object.

…he previous code repopened the file with `open()` that has `rt` the new file must also be opened that way.

…nction