When I originally rewrote Servo's mach bootstrapping (using virtualenv
w/ requirements.txt in #7103), I didn't specify mach as a requirement
because a new version hadn't been published in a while. Now that 0.6
is out, I asked the mach maintainers to publish a new version on PyPI,
so now we can fetch it like the other Python dependencies.
Fixes https://github.com/servo/servo/issues/10728.