bindings: python: examples: add graceful exit to async_watch_line_value

bindings: python: examples: add graceful exit to async_watch_line_value

The purpose of the example is demonstrate using a request with poll().
It provides a hint as to how the poll can be combined with other fds but,
as Python comes with batteries included, the Python version of the example
can be readily extended to actually demonstrate this, as well as how it
can be used in multi-threaded environments.

Extend the example to use an eventfd to allow the poll() to be run in
a background thread and be gracefully terminated by the main thread.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240917125455.324551-1-warthog618@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>