[code-quality] Potential problems with tox --parallel and forking code-quality tools

Hi all, I noticed https://github.com/tox-dev/tox/issues/1183 and I suspect the problem is in how `tox` and `trial` are using sub-processes for parallel work. I know that Flake8 uses sub-processes as well (via multiprocessing) so I'd be unsurprised if this eventually shows up in Flake8's issue tracker. I don't recall how pylint does parallel processing, but if it's anything like Flake8, I'm guessing they might see it soon too for large enough code-bases. I wanted to give y'all a heads up in case you notice or get reports about this. Cheers, Ian