Don't stop the loop in run_until_complete() on SystemExit and KeyboardInterrupt exceptions

[Jackenmen]

Fixes #336

[1st1]

Can you add a unittest?

[Jackenmen]

@1st1 I added the unit test based on CPython's equivalent and rebased the PR's branch on top of master.
Let me know if there's anything else you would want me to do.

[Jackenmen]

I've rebased this on top of master since it looks like there's a new required check (although now I see it doesn't seem to work?) + it's been moved to the project board it seems. I'm still available to do any additional changes if needed so just let me know if needed.

I'm also mentioning this since I'm hoping this could get included in the next release, whenever it's ready of course :)

[fantix]

Thanks for the PR! The thing is the test is NOT failing without the fix, I'm looking into it now.

UPDATE: Looks like this cpython test was added before _StopError was removed. Because current asyncio will complete running the whole _ready queue before actually stopping the loop, the func() is always called. So we should fix the test both here and in asyncio.