bpo-39877: Refactor take_gil() function (GH-18885)

* Remove ceval parameter of take_gil(): get it from tstate.
* Move exit_thread_if_finalizing() call inside take_gil(). Replace
  exit_thread_if_finalizing() with tstate_must_exit(): the caller is
  now responsible to call PyThread_exit_thread().
* Move is_tstate_valid() assertion inside take_gil(). Remove
  is_tstate_valid(): inline code into take_gil().
* Move gil_created() assertion inside take_gil().