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

Activities marked final using Sorbet's `final!` fail on registration, with something like:
```
RuntimeError:
# 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).