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/_sqlite/connection.h b/Modules/_sqlite/connection.h
index 2860a0c..5fb410a 100644
--- a/Modules/_sqlite/connection.h
+++ b/Modules/_sqlite/connection.h
@@ -61,7 +61,7 @@
     int initialized;
 
     /* thread identification of the thread the connection was created in */
-    long thread_ident;
+    unsigned long thread_ident;
 
     pysqlite_Cache* statement_cache;