Fix the wrong signature of stackless.get_schedule_callback

I'm sorry, but my commit d1f907bd contains a little copy and paste error. 
This pull request fixes the error.

pydevd monkey patches the function stackless.get_schedule_callback.
Previously the patched get_schedule_callback had a different signature
than the original function. See https://stackless.readthedocs.org/en/latest/library/stackless/stackless.html#stackless.get_schedule_callback