Fix cast and minor edits to test

Author: colesbury

Lib/test/test_free_threading/test_collections.py

@@ -24,8 +24,7 @@ def copy_loop():
             for _ in range(1000):
                 copy(d)
 
-        workers = [mutate, mutate, copy_loop, copy_loop]
-        threading_helper.run_concurrently(workers)
+        threading_helper.run_concurrently([mutate, copy_loop])
 
 
 if __name__ == "__main__":

Modules/_collectionsmodule.c

@@ -1991,7 +1991,7 @@ dequeiter_next(PyObject *op)
     // it mid-iteration. The one-object critical section avoids this
     // because it keeps the deque locked across calls when it's already
     // held, due to a fast-path optimization.
-    if (_PyObject_IsUniquelyReferenced(it)) {
+    if (_PyObject_IsUniquelyReferenced((PyObject *)it)) {
         Py_BEGIN_CRITICAL_SECTION(deque);
         result = dequeiter_next_lock_held(it, deque);
         Py_END_CRITICAL_SECTION();