Run tests on py38 as well

[gsalgado]

No description provided.

[gsalgado]

Several tests seem to hang forever, but if you try to run them with -s --log-cli-level=debug they pass. I'm not looking forward to dig into that, so if anybody wants to have a go, be my guest ;-)

[gsalgado]

The hangs seem to have been caused by us hitting a race condition when we kill the process before it has had a chance to actually start executing -- a really short sleep before proc.send_signal() did the trick. A few tests fail randomly on py38, though -- I've left XXX comments on those. And one still hangs forever

[gsalgado]

With #24 most tests are passing without the need for any changes -- there are only 3 tests that very rarely fail

[gsalgado]

The tests failing randomly are not exclusive to py38 -- I can reproduce that in py37 as well, and filed #25 for that

[gsalgado]

@pipermerriam I'm going ahead and merging this without waiting for a review as it doesn't include any changes to code/tests anymore