Thank you Brett, the problem was the missed exec(). With the patch in attachment the tests pass, but it does not seem to me a good solution. Infact, changing the filter at runtime has no effect: 

$ cat foo.py 
import warnings

warnings.simplefilter("always")
async = 33
await = 33

$ ./python foo.py 
$

Does this happen because, putting the PyErr_WarnEx() in Python/ast.c, the warning is issued before the runtime?

Furthermore, if I set the filter from the CL, then the warning is properly triggered, but the file name and line number are wrong: 

$ ./python -Wd foo.py 
sys:1: DeprecationWarning: 'async' will become a reserved keyword in Python 3.7
sys:1: DeprecationWarning: 'await' will become a reserved keyword in Python 3.7