Remove legacy synchroneus import

tests/test_inbox_locks.py

@@ -14,3 +14,19 @@ def test_inbox_run_locks_serialize_different_inboxes():
     second = locks.acquire("second", blocking=False)
     assert second is not None
     second.release()
+
+
+def test_inbox_run_locks_serialize_across_lock_instances():
+    first_locks = InboxRunLocks()
+    second_locks = InboxRunLocks()
+    first = first_locks.acquire("first", blocking=False)
+    assert first is not None
+
+    try:
+        assert second_locks.acquire("second", blocking=False) is None
+    finally:
+        first.release()
+
+    second = second_locks.acquire("second", blocking=False)
+    assert second is not None
+    second.release()