bpo-1635741: Refactor _threadmodule.c (GH-23793)

* Fix ExceptHookArgsType name: "_thread.ExceptHookArgs", instead of
  "_thread._ExceptHookArgs".
* PyInit__thread() no longer intializes interp->num_threads to 0:
  it is already done in PyInterpreterState_New().
* Use PyModule_AddType(), Py_NewRef() and Py_XNewRef().
* Replace str_dict variable with _Py_IDENTIFIER(__dict__).
* Remove assert(Py_IS_TYPE(obj, &Locktype)) from release_sentinel()
  to avoid having to retrive the type from this callback.
* Add thread_bootstate_free()
* Rename t_bootstrap() to thread_run()
* bootstate structure: rename keyw member to kwargs

Author: vstinner