bpo-31370: Remove support for threads-less builds (#3385)

* Remove Setup.config
* Always define WITH_THREAD for compatibility.

Lib/test/test_imaplib.py

@@ -1,8 +1,4 @@
 from test import support
-# If we end up with a significant number of tests that don't require
-# threading, this test module should be split.  Right now we skip
-# them all if we don't have threading.
-threading = support.import_module('threading')
 
 from contextlib import contextmanager
 import imaplib
@@ -10,6 +6,7 @@ import os.path
 import socketserver
 import time
 import calendar
+import threading
 
 from test.support import (reap_threads, verbose, transient_internet,
                           run_with_tz, run_with_locale, cpython_only)