EDIT: As mentioned above, this isn't the Ellipsis object, but the result of a looped list. I jumped the gun here. Knowing about the Ellipsis object is a good bit of back shelf knowledge should you find an Ellipsis in some actual code, rather than the output.
The Ellipsis object in Python is used for extended slice notation. It's not used in current Python core libraries, but is available for developers to define in their own libraries. For example, NumPy (or SciPy) use this as part of their array object. You'll need to look at the documentation for tree() to know exactly how Ellipsis behaves in this object.
From Python documentation:
3.11.8 The Ellipsis Object
This object is used by extended slice notation (see the Python Reference Manual). It supports no special operations. There is exactly one ellipsis object, named Ellipsis (a built-in name).
It is written as Ellipsis.