Fix hang on shutdown when asyncio.create_subprocess is used

[cdleonard]

Fixes #41 - can now using asyncio.create_subprocess while coverage_sh is enabled.

As far as I can tell asyncio.create_subprocess will initialize a subprocess.Popen object internally and use it with a custom PID waiter, skipping Popen.wait and Popen.poll. I was only able to get this to work by hooking into Popen.__del__.

There are other use cases where hooking Popen.__del__ is useful, for example if the user simply leaks a subprocess.Popen that shouldn't cause coverage_sh threads to simple hang.

It might make sense to also mark CoverageParserThread as a daemon thread. This shouldn't replace the __del__ hook because then threads would leak until main process exit.

Includes a new test_end2end_async_simple which does coverage on a new main_async.py script which calls asyncio.create_subprocess. Without PatchedPopen.__del__ it hangs - demonstrating the original issue. It also surprisingly shows that coverage works as expected!

[lackhove]

Very cool, thanks! I will probably look into this next weekend. FYI: #40 and #45 will certainly conflict with all previous PRs, so i would like to get this one (and maybe #42) merged first.

[cdleonard]

Removed dependency on #42

[lackhove]

@cdleonard awesome, thank you!