With Sorbet, can't make an activity or workflow class final

[viettrungluu]

Activities marked final using Sorbet's final! fail on registration, with something like:

RuntimeError:
  #<Class:0x00005558e0b25190> was declared as final but its method `execute` was not declared as final

(We/I like final classes, to prevent inadvertent implementation inheritance.)

This was broken by #128, which added the dup.

Minimal repro looks something like (off the top of my head):

# typed: strict

class MyActivity < Temporal::Activity
  extend T::Sig
  extend T::Helpers

  final!

  sig(:final) { void }
  def execute; end
end

(and then trying to register the activity).