__bases__ is documented, but __base__ is not.

$ grep -r __base__ Doc
$ grep -r __bases__ Doc
Doc/c-api/object.rst:are different objects, *B*'s :attr:`~class.__bases__` attribute is searched in
Doc/c-api/object.rst:a depth-first fashion for *A* --- the presence of the :attr:`~class.__bases__`
Doc/extending/newtypes.rst:   in its :attr:`~class.__bases__`, or else it will not be able to call your type's
Doc/library/email.headerregistry.rst:    class's ``__bases__`` list.
Doc/library/functions.rst:   tuple itemizes the base classes and becomes the :attr:`~class.__bases__`
Doc/library/stdtypes.rst:.. attribute:: class.__bases__
Doc/reference/datamodel.rst:      single: __bases__ (class attribute)
Doc/reference/datamodel.rst:   dictionary containing the class's namespace; :attr:`~class.__bases__` is a
Doc/whatsnew/2.3.rst:  removed: you can now assign to the :attr:`__name__` and :attr:`__bases__`
Doc/whatsnew/2.3.rst:  assigned to :attr:`__bases__` along the lines of those relating to assigning to