add error case found by CPython test "test_await_15" in test_coroutines_pep492
add error case found by CPython test "test_await_15" in test_coroutines_pep492

cython/cython,da-woods/cython,scoder/cython,scoder/cython,scoder/cython,da-woods/cython,da-woods/cython,da-woods/cython,cython/cython,cython/cython,cython/cython,scoder/cython