Fixes timer global init of event handler

[pbaughman]

Fixes #183

• Use an attribute on the context to see if we need to install the timer
  callback event handler
• Add a regression test that fails (hangs, actually) before the fix was applied

I manually verified that a launch description containing multiple timers would only register the TimerEvent handler once.

[wjwwood]

Other than one small implementation detail, this lgtm.

[wjwwood]

CI:

• Linux
• Linux-aarch64
• macOS
• Windows

[pbaughman]

Hah - my different version of flake8 strikes again. . .

./test/launch/test_timer_action.py:19:1: E302 expected 2 blank lines, found 1
def test_multiple_launch_with_timers():
^

./test/launch/test_timer_action.py:27:24: Q000 Remove bad quotes
                period="1",
                       ^

./test/launch/test_timer_action.py:29:52: Q000 Remove bad quotes
                    launch.actions.Shutdown(reason="Timer expired")
                                                   ^

./launch/actions/timer_action.py:138:33: Q000 Remove bad quotes
        if not hasattr(context, "_TimerAction__event_handler_has_been_installed"):
                                ^

./launch/actions/timer_action.py:148:30: Q000 Remove bad quotes
            setattr(context, "_TimerAction__event_handler_has_been_installed", True)
                             ^

1     E302 expected 2 blank lines, found 1
4     Q000 Remove bad quotes

143 files checked
5 errors

'E'-type errors: 1
'Q'-type errors: 4

[wjwwood]

That might also be that you're missing some of the flake8 extensions we use, but I'm not sure. I think the quote style one is an extension (again not 100% sure, but I think I remember that being the case).

[wjwwood]

I'll rerun CI quickly if you can fix those up, so we can unblock #181.

[wjwwood]

CI:

• Linux
• Linux-aarch64
• macOS
• Windows