kick off two processes that need to provision the tox environment and
ensure they don't stomp on each other in the process.

avoid race condition when `.tox` environment needs to be provisioned

subprocess.Popen didn't accept the `encoding` kwarg.

rather than attempting other tricks, just match the output as an
explicit bytestring, which will work on any interpreter.

as per PR checklist

add changelog information about issue tox-dev#2515

rst ``code`` is two backticks.................................Failed
- hook id: rst-backticks
- exit code: 1