Exclude singleton classes from subclasses and descendants

This behavior changed in Ruby starting with 2.3.0, as a result of
https://bugs.ruby-lang.org/issues/11360. This results in a change in
behavior of these methods which is likely undesirable.

Fixes rails#27238

Author: sgrif

activesupport/lib/active_support/core_ext/class/subclasses.rb

@@ -22,6 +22,7 @@ class Class
     def descendants
       descendants = []
       ObjectSpace.each_object(singleton_class) do |k|
+        next if k.singleton_class?
         descendants.unshift k unless k == self
       end
       descendants

activesupport/test/core_ext/class_test.rb

@@ -25,4 +25,14 @@ def test_subclasses
     assert_equal [Baz], Bar.subclasses
     assert_equal [], Baz.subclasses
   end
+
+  def test_descendants_excludes_singleton_classes
+    klass = Parent.new.singleton_class
+    refute Parent.descendants.include?(klass), "descendants should not include singleton classes"
+  end
+
+  def test_subclasses_excludes_singleton_classes
+    klass = Parent.new.singleton_class
+    refute Parent.subclasses.include?(klass), "subclasses should not include singleton classes"
+  end
 end