bpo-39520: Fix un-parsing of ext slices with no dimensions (GH-18304)

(cherry picked from commit 185903de12de8837bf0dc0008a16e5e56c66a019)

Co-authored-by: Batuhan Taşkaya <47358913+isidentical@users.noreply.github.com>
diff --git a/Lib/test/test_future.py b/Lib/test/test_future.py
index fd468b5..d83c47e 100644
--- a/Lib/test/test_future.py
+++ b/Lib/test/test_future.py
@@ -256,6 +256,11 @@
         eq("slice[:-1]")
         eq("slice[1:]")
         eq("slice[::-1]")
+        eq("slice[:,]")
+        eq("slice[1:2,]")
+        eq("slice[1:2:3,]")
+        eq("slice[1:2, 1]")
+        eq("slice[1:2, 2, 3]")
         eq("slice[()]")
         eq("slice[a, b:c, d:e:f]")
         eq("slice[(x for x in a)]")