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

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

diff --git a/Lib/reprlib.py b/Lib/reprlib.py
index 40d991f..616b343 100644
--- a/Lib/reprlib.py
+++ b/Lib/reprlib.py
@@ -4,10 +4,7 @@
 
 import builtins
 from itertools import islice
-try:
-    from _thread import get_ident
-except ImportError:
-    from _dummy_thread import get_ident
+from _thread import get_ident
 
 def recursive_repr(fillvalue='...'):
     'Decorator to make a repr function return fillvalue for a recursive call'