bpo-6532: Make the thread id an unsigned integer. (#781)

* bpo-6532: Make the thread id an unsigned integer.

From C API side the type of results of PyThread_start_new_thread() and
PyThread_get_thread_ident(), the id parameter of
PyThreadState_SetAsyncExc(), and the thread_id field of PyThreadState
changed from "long" to "unsigned long".

* Restore a check in thread_get_ident().
diff --git a/Lib/_dummy_thread.py b/Lib/_dummy_thread.py
index 36e5f38..a2cae54 100644
--- a/Lib/_dummy_thread.py
+++ b/Lib/_dummy_thread.py
@@ -69,7 +69,7 @@
     available, it is safe to assume that the current process is the
     only thread.  Thus a constant can be safely returned.
     """
-    return -1
+    return 1
 
 def allocate_lock():
     """Dummy implementation of _thread.allocate_lock()."""