Having a type checker run before the Python interpreter in our
current day continuous build/integration environment adds a
second step

This is very similar to statically typed languages. They also have two steps:

• There is the first step that checks the types but does not run the program. In C/C++, this is the compiler, in Python it is "mypy".
• There is the second step that runs the program but does not check the types. In C/C++, this is running the executable, in Python it is "python".

In Python, we can skip the first step, and perhaps that is the source of your concern.  I doubt anyone that has adopted type hinting in Python will forget to run mypy, especially in a CI environment like you consider.

--Ned.