Adding examples as pytest tests

- This way they will be included in CI checks
- Decreased the delay time in the thread-like examples to speed up tests
(probably could decrease the time some more)
- Added an async version of the calculator test
- Forcing python3 support for example scripts

examples/addressbook.py

@@ -1,3 +1,5 @@
+#!/usr/bin/env python3
+
 from __future__ import print_function
 import capnp # noqa: F401