If you count pre-release versions, there are actually 7 pre-declared constants, since Python 3.15 (planned for release in November [0]) adds a new constant "TYPE_CHECKING" that should behave like "Ellipsis" and "NotImplemented" do right now [1].

[0]: https://peps.python.org/pep-0790/#schedule

[1]: https://peps.python.org/pep-0781/#backwards-compatibility