bpo-35207: Disallow expressions like (a) = 42

[pablogsal]

As commented initially in bpo-33878, the fact that expressions like
(x) = 42 are accepted is, in reality, an implementation detail and
they should be disallowed.

I am not sure if this is the cleanest/simplest implementation...if you think there is a better way to do this I am happy to change the Pull Request :)

https://bugs.python.org/issue35207

[pablogsal]

I am not sure what to do with the annotated assignment as in test_grammar there are these uses:

exec('X: int; Z: str = "Z"; (w): complex = 1j', ns)

def f2bad():
    (no_such_global): int
    print(no_such_global)

[gvanrossum]

We should not do this. See the issue.