IIRC Mypy started off as a type annotation syntax and corresponding type checker for Python. Mypy's type annotations were adopted by Python itself (in version 3.5 - PEP 484), which reduced Mypy's role to be just a type checker.

Since then, type annotations have indeed become a core part of Python - not only are they used in frameworks and libraries, but are also required to use language features like @dataclass.