`pip` timeouts can cause complete deletion of package directory contents on Windows

[chennes]

For background see FreeCAD/FreeCAD#25810

The Addon Manager calls pip with a predefined timeout. This is problematic for several reasons:

1. The main mechanism for doing the update does not update the UI with any kind of indication that it's still going, and the timeout is now very long.
2. It's not cancellable by the user, even though the underlying call is, so it could be.
3. On Windows, when the pip --upgrade call is terminated by a timeout, it deletes the contents of the target directory. So we need to make a backup before beginning. This would be a good idea anyway, so if the upgrade fails it can be dealt with.