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