gh-136434: Fix docs generation of UnboundItem in subinterpreters (#136435)

Lib/concurrent/interpreters/_crossinterp.py

@@ -40,16 +40,21 @@ class UnboundItem:
 
     @classonly
     def singleton(cls, kind, module, name='UNBOUND'):
-        doc = cls.__doc__.replace('cross-interpreter container', kind)
-        doc = doc.replace('cross-interpreter', kind)
+        doc = cls.__doc__
+        if doc:
+            doc = doc.replace(
+                'cross-interpreter container', kind,
+            ).replace(
+                'cross-interpreter', kind,
+            )
         subclass = type(
             f'Unbound{kind.capitalize()}Item',
             (cls,),
-            dict(
-                _MODULE=module,
-                _NAME=name,
-                __doc__=doc,
-            ),
+            {
+                "_MODULE": module,
+                "_NAME": name,
+                "__doc__": doc,
+            },
         )
         return object.__new__(subclass)
 

Misc/NEWS.d/next/Library/2025-07-08-20-58-01.gh-issue-136434.uuJsjS.rst

@@ -0,0 +1,2 @@
+Fix docs generation of ``UnboundItem`` in :mod:`concurrent.interpreters`
+when running with :option:`-OO`.