[ruby/pp] Use .class.members for pretty printing Data

* Data#members might not be defined, instead it might be defined
  on Data subclasses or a module included there. This is notably the
  case on TruffleRuby which defines it there for optimization purposes.
  In fact the mere presence of Data#members implies a megamorphic call
  inside, so it seems best to avoid relying on its existence.

https://github.com/ruby/pp/commit/6a97d36fbb

lib/pp.rb

@@ -423,7 +423,7 @@ end
 class Data # :nodoc:
   def pretty_print(q) # :nodoc:
     q.group(1, sprintf("#<data %s", PP.mcall(self, Kernel, :class).name), '>') {
-      q.seplist(PP.mcall(self, Data, :members), lambda { q.text "," }) {|member|
+      q.seplist(PP.mcall(self, Kernel, :class).members, lambda { q.text "," }) {|member|
         q.breakable
         q.text member.to_s
         q.text '='