Fix event loop closed bug

[hidmic]

Fixes #185. Apparently, atexit registered functions get run before the python's gc runs objects destructors and thus asyncio loops were being closed before ExecuteProcess related awaitables were terminated.

This PR also fixes an issue introduced by #167 on Python 3.5 (in Xenial).

[dirk-thomas]

This PR also fixes an issue introduced by #167 on Python 3.5 (in Xenial).

This looks like a duplicate of #196 and shouldn't be necessary since Xenial / Python 3.5 isn't a supported platform on master anymore.

[hidmic]

Fair enough, I'll drop that commit.

[wjwwood]

This patch is definitely more defensive, but I'd be interested to see if the atexit handler is even needed anymore, see:

launch/launch/launch/launch_service.py

Lines 52 to 55 in 632ef1f

	# This atexit handler ensures all the loops are closed at exit.
	# This is only really required pre-3.6, see:
	# https://github.com/ros2/launch/issues/84
	# https://bugs.python.org/issue23548

[hidmic]

This patch is definitely more defensive, but I'd be interested to see if the atexit handler is even needed anymore

It is on Python 3.5, that bug is still there. One could argue, as Dirk did, that we're not targeting Ubuntu Xenial anymore and (transitively) neither Python 3.5, and thus we should probably drop that handler. I didn't do it to keep this PR small and the discussion brief :)