What PEP 484 was standardize the use of annotations as type annotations and provide ancillary out-of-the-box support for type hinting via annotations, particularly the typing module.

Mypy was actually using python 3.x annotation for type annotations before PEP 484 standardized them brought the stdlib typing module, but with PEP 484 there was a common, language-defined standard baseline for mypy and other efforts.