Adopt PEP 484 type hints for C++ types exported to Python
diff --git a/example/example-callbacks.py b/example/example-callbacks.py
index 674174a..1d18c89 100755
--- a/example/example-callbacks.py
+++ b/example/example-callbacks.py
@@ -82,3 +82,6 @@
         print("All OK!")
     else:
         print("Problem!")
+
+print(test_callback3.__doc__)
+print(test_callback4.__doc__)