> A utility to check whether an AST requires async mode should be fairly straightforward.

Here is one case we forgot in IPython apparently :

In [1]: x = 1
   ...: def f():
   ...:     nonlocal x
   ...:     x = 10000

This is not detected as a syntax error, but considered as asyn, we took an approach that prefer false positive (try to run invalid syntax as async) than false negative (reject valid async-code). Add this to the test suite for this feature.