bpo-1635741: Refactor _threadmodule.c

[vstinner]

• 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.

https://bugs.python.org/issue1635741

[vstinner]

I chose to not convert static types to heap types in the same PR to keep the PR short enough for reviews.

cc @corona10 @shihai1991

[vstinner]

I was not sure if it's a good idea to convert the module to multiphase init, but I wanted to do it to pass the lock type to newlockobject().

[vstinner]

I was not sure if it's a good idea to convert the module to multiphase init, but I wanted to do it to pass the lock type to newlockobject().

I changed my mind. I reverted some changes to keep the legacy PyModule_Create() API. I removed the module state.

It's trivial to add a module state to migrate to PyModuleDef_Init(). But we may do that while static types are converted to heap types.

[vstinner]

The tricky part is to get localdummytype in local_new() and get localtype in local_getattro(). _PyType_GetModuleByDef() can be used for that.