Function PyCriticalSection2_BeginMutex 

pub unsafe extern "C" fn PyCriticalSection2_BeginMutex(
    c: *mut PyCriticalSection2,
    m1: *mut PyMutex,
    m2: *mut PyMutex,
)

Available on non-Py_LIMITED_API and Py_3_13 and Py_3_14 only.