check if the attribute is set before deleting it with T_OBJECT_EX (fixes #7604)

Also, add a note to the docs about the better behavior of T_OBJECT_EX as
compared to T_OBJECT.
diff --git a/Misc/NEWS b/Misc/NEWS
index 3098505..87242b1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- Issue #7604: Deleting an unset slotted attribute did not raise an
+  AttributeError.
+
 - Issue #7534: Fix handling of IEEE specials (infinities, nans,
   negative zero) in ** operator.  The behaviour now conforms to that
   described in C99 Annex F.