bindings: python: change the interpretation of None in event wait

bindings: python: change the interpretation of None in event wait

The docs don't mention it but currently passing None as the timeout to
one of the event wait methods works like passing 0 to select() - the wait
method returns immediately. Change it to a more standard behavior - None
makes the method block indefinitely until an event becomes available for
reading.

This is a slight change in the behavior but let's hope nobody complains
as libgpiod v2 is still pretty recent and its adoption is (hopegully)
not wide-spread yet.

Reported-by: Nicolas Frattaroli <frattaroli.nicolas@gmail.com>
Suggested-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Kent Gibson <warthog618@gmail.com>