I've been using enforce for this purpose (https://github.com/RussBaz/enforce). It provides decorators that enable type checking where wanted. This gives the advantage of migrating existing code to type checking at your own pace at least.

On a general note I've come across a few cases, when pushing python's type annotations to their limits, that force you to put the type names in string quotes, and that makes the whole thing feel like a hack. It's better than nothing for my use cases, but if I remember correctly both mypy and enforce have problems in common that are probably coming from the way python itself is built, such as self reference in a class definition.