Make return type of functools.cache_property covariant (#10053)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

test_cases/stdlib/check_functools.py

@@ -46,3 +46,22 @@ def check_cached_property_settable(x: int) -> None:
     assert_type(b.x, int)
     b.x = x
     assert_type(b.x, int)
+
+
+# https://github.com/python/typeshed/issues/10048
+class Parent: ...
+
+
+class Child(Parent): ...
+
+
+class X:
+    @cached_property
+    def some(self) -> Parent:
+        return Parent()
+
+
+class Y(X):
+    @cached_property
+    def some(self) -> Child:  # safe override
+        return Child()