Fix SF buf #480096: Assign to __debug__ still allowed

Easy enough to catch assignment in the compiler.  The perverse user
can still change the value of __debug__, but that may be the least he
can do.
diff --git a/Lib/test/output/test_compile b/Lib/test/output/test_compile
index de33352..303b745 100644
--- a/Lib/test/output/test_compile
+++ b/Lib/test/output/test_compile
@@ -1,5 +1,4 @@
 test_compile
-testing complex args
 1 2
 1 2
 3 4