Adding thread state storage for embedders  #1215

Environment

• Pythonnet version: 3.0.0
• Python version: 3.x
• Operating System: Windows 10, + others

Details

• I need access to the PyThreadState_GetDict cpython call.

  I have embedded Python.Net in my app but since I have multiple threads (tasks) running scripts and calling back into C#, I require a way to store my thread context into the current Python thread.
  To to this I imported PyThreadState_GetDict myself (worked fine), but I also needed the other low level cpython calls that are set as internal in the Python.Runtime class.

So here would be the new member exposed by Python.Runtime (or somewhere)

public PyDict GetThreadStateDict()
{
	IntPtr dict = Runtime.PyThreadState_GetDict();
	Runtime.XIncref(dict); // borrowed ref
	return new PyDict(dict);
}

And, then we can get & set an object of some kind

protected ScriptContext GetContext()
{
	using (var tdict = GetThreadStateDict())
	{
		using (PyObject context_key = tdict.GetItem(_thread_cookie_name_s))
		{
			return (ScriptContext)context_key.AsManagedObject(typeof(ScriptContext));
		}
	}
}

protected void SetContext(ScriptContext context)
{
	using (var tdict = GetThreadStateDict())
	{
		tdict.SetItem(_thread_cookie_name_s, context.ToPython());
	}
}

That is, unless there's a better way to keep track of what original thread is calling back into the app.
Maybe the Python Thread Specific Storage (TSS) API would be better?