Typing annotations of variables used in launch only available for python 3.6+

[dhood]

http://mypy.readthedocs.io/en/latest/cheat_sheet_py3.html#variables

causes failures on xenial jobs, e.g.:

    from .launch_context import LaunchContext
E     File "/home/rosbuild/ci_scripts/ws/install/launch/lib/python3.5/site-packages/launch/launch_context.py", line 45
E       self._event_queue: asyncio.Queue = asyncio.Queue()
E                        ^
E   SyntaxError: invalid syntax

[dhood]

@wjwwood putting this here for tracking/visibility, not trying to dump it and leave it. we can group-schedule it when it reaches priority.