Bug #1588287: fix invalid assertion for `1,2` in debug builds.

Will backport
diff --git a/Python/ast.c b/Python/ast.c
index f490268..dcf7f2b 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -532,6 +532,7 @@
            || TYPE(n) == listmaker
            || TYPE(n) == testlist_gexp
            || TYPE(n) == testlist_safe
+           || TYPE(n) == testlist1
            );
 
     seq = asdl_seq_new((NCH(n) + 1) / 2, c->c_arena);