bpo-39877: Fix PyEval_RestoreThread() for daemon threads

[vstinner]

• exit_thread_if_finalizing() does now access directly _PyRuntime
  variable, rather than using tstate->interp->runtime since tstate
  can be a dangling pointer after Py_Finalize() has been called.
• exit_thread_if_finalizing() is now called before calling
  take_gil(). There is no need to protect the call with the GIL.
• Add ENSURE_TSTATE_NOT_NULL() macro to check that tstate is not NULL
  at runtime. Check tstate earlier. take_gil() is now responsible to
  check that tstate is not NULL.

Cleanup:

• PyEval_RestoreThread() no longer saves/restores errno: it's already
  done inside take_gil().
• PyEval_AcquireLock(), PyEval_AcquireThread(),
  PyEval_RestoreThread() and _PyEval_EvalFrameDefault() now check if
  tstate is valid with the new is_tstate_valid() function which uses
  _PyMem_IsPtrFreed().

https://bugs.python.org/issue39877