fix ctypes test alignment assumptions (closes #20946)

Patch by Andreas Schwab.
diff --git a/Misc/NEWS b/Misc/NEWS
index 99a6cf1..e2c757c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -271,6 +271,8 @@
 Tests
 -----
 
+- Issue #20946: Correct alignment assumptions of some ctypes tests.
+
 - Issue #20743: Fix a reference leak in test_tcl.
 
 - Issue #20510: Rewrote test_exit in test_sys to match existing comments,