Fix Formula.factory for Formula subclasses

Fixes Homebrew/homebrew#16288.

Library/Homebrew/formula.rb

@@ -395,7 +395,7 @@ class Formula
       raise LoadError
     end
 
-    raise NameError if klass.superclass != Formula
+    raise NameError if !klass.ancestors.include? Formula
 
     return klass.new(name) if install_type == :from_name
     return klass.new(name, path.to_s)