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/Modules/_ssl.c b/Modules/_ssl.c
index 421e0b6..327fb37 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -5032,8 +5032,7 @@
 static void
 _ssl_threadid_callback(CRYPTO_THREADID *id)
 {
-    CRYPTO_THREADID_set_numeric(id,
-                                (unsigned long)PyThread_get_thread_ident());
+    CRYPTO_THREADID_set_numeric(id, PyThread_get_thread_ident());
 }
 #else
 /* deprecated CRYPTO_set_id_callback() API. */