bpo-36052: Raise a SyntaxError when assigning a value to __debug__ with := (GH-11958)

Trying to assign a value to __debug__ using the assignment operator is supposed to fail, but
a missing check for forbidden names when setting the context in the ast was preventing this behaviour. 

Lib/test/test_syntax.py

@@ -51,6 +51,10 @@ SyntaxError: cannot assign to __debug__
 Traceback (most recent call last):
 SyntaxError: cannot assign to __debug__
 
+>>> (__debug__ := 1)
+Traceback (most recent call last):
+SyntaxError: cannot assign to __debug__
+
 >>> f() = 1
 Traceback (most recent call last):
 SyntaxError: cannot assign to function call

Misc/NEWS.d/next/Core and Builtins/2019-02-20-17-57-31.bpo-36052.l8lJSi.rst

+Raise a :exc:`SyntaxError` when assigning a value to `__debug__` with the
+Assignment Operator. Contributed by Stéphane Wirtel and Pablo Galindo.

Python/ast.c

@@ -1084,7 +1084,7 @@ set_context(struct compiling *c, expr_ty e, expr_context_ty ctx, const node *n)
                 return 0;
             break;
         case Name_kind:
-            if (ctx == Store) {
+            if (ctx == Store || ctx == NamedStore) {
                 if (forbidden_name(c, e->v.Name.id, n, 0))
                     return 0; /* forbidden_name() calls ast_error() */
             }