remove redundant check and add Carl's other test

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/slots.md

@@ -203,6 +203,20 @@ class B(A):
 class C(B, A): ...  # fine
 ```
 
+The same principle, but a more complex example:
+
+```py
+class AA:
+     __slots__ = ("a",)
+
+class BB(AA):
+    __slots__ = ("b",)
+
+class CC(BB): ...
+class DD(AA): ...
+class FF(CC, DD): ...  # fine
+```
+
 ## False negatives
 
 ### Possibly unbound

crates/ty_python_semantic/src/types/class.rs

@@ -458,12 +458,12 @@ impl<'db> ClassType<'db> {
             return true;
         }
 
-        if self.is_subclass_of(db, other) || other.is_subclass_of(db, self) {
-            return true;
+        // Optimisation: if either class is `@final`, we only need to do one `is_subclass_of` call.
+        if self.is_final(db) {
+            return self.is_subclass_of(db, other);
         }
-
-        if self.is_final(db) || other.is_final(db) {
-            return false;
+        if other.is_final(db) {
+            return other.is_subclass_of(db, self);
         }
 
         // Two solid bases can only coexist in an MRO if one is a subclass of the other.