You are slightly wrong.

Python does allow you to put anything in annotations. ( pep 3107 that defines type annotations says that explicitly [1]).

But it also defines a type checking annotations, which is a specific convention for using pep 3107 annotations. Type annotations were introduced in PEP 484 and updated in a lot of subsequent peps. The python typing system is fully specified in [3].

It does have several implementations, although the reference implementation is mypy

[1]: https://peps.python.org/pep-3107/#rejected-proposals

[2]: https://peps.python.org/pep-0484/

[3]: https://typing.python.org/en/latest/spec/index.html