Neither Python example was registered as a test, so `ctest -L examples`
in CI skipped them entirely -- which is how 08's missing Python node
went unnoticed.
Add a kpn_python_example() helper (gated on KPN_BUILD_PYTHON) that runs
each script with PYTHONPATH pointed at the freshly-built module, so it
does not depend on cwd or a hard-coded build/python path, and register
07 and 08. Also del the network in 07 for deterministic teardown.
