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/Include/pythread.h b/Include/pythread.h
index 88c4873..dbacb8b 100644
--- a/Include/pythread.h
+++ b/Include/pythread.h
@@ -17,10 +17,14 @@
     PY_LOCK_INTR
 } PyLockStatus;
 
+#ifndef Py_LIMITED_API
+#define PYTHREAD_INVALID_THREAD_ID ((unsigned long)-1)
+#endif
+
 PyAPI_FUNC(void) PyThread_init_thread(void);
-PyAPI_FUNC(long) PyThread_start_new_thread(void (*)(void *), void *);
+PyAPI_FUNC(unsigned long) PyThread_start_new_thread(void (*)(void *), void *);
 PyAPI_FUNC(void) PyThread_exit_thread(void);
-PyAPI_FUNC(long) PyThread_get_thread_ident(void);
+PyAPI_FUNC(unsigned long) PyThread_get_thread_ident(void);
 
 PyAPI_FUNC(PyThread_type_lock) PyThread_allocate_lock(void);
 PyAPI_FUNC(void) PyThread_free_lock(PyThread_type_lock);