I added the PyErr_WarnEx(PyExc_DeprecationWarning, ...) in Python/ast.c, right below the check for None, True and False as names. Running the following test the message is properly printed:

def test_async(self):
    with self.assertWarnsRegex(DeprecationWarning, "reserved keyword"):
        async = 33

However, the test does not pass, because the DeprecationWarning is not triggered. I am sorry but as a cpython beginner I can not figure out how to solve the problem, so I hope someone else can find the time to do it