make struct sequences subclass tuple; kill lots of code

This fixes #8413.
diff --git a/Misc/NEWS b/Misc/NEWS
index 0022a4e..9eede94 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,8 @@
 Core and Builtins
 -----------------
 
+- Issue #8413: structsequence now subclasses tuple.
+
 - Issue #8271: during the decoding of an invalid UTF-8 byte sequence, only the
   start byte and the continuation byte(s) are now considered invalid, instead
   of the number of bytes specified by the start byte.