It seems that letting CircleCI use its default docker version (17.09.0-ce,
apparently) did not interact well with multiarch builds: in particular, we saw
weird effects where running an amd64 build at the same time as an arm64 build
caused the arm64 builds to fail with:
Error while loading /usr/sbin/dpkg-deb: No such file or directory
* remove 2.7 from CI and publishing
* fill out classifiers and also make it not be installed on 3.5
* some minor bumps so that the old deps work on python 3.5
Latest is horrible and makes debugging what has happened anywhere a
nightmare. We push a latest because of demand for it, but we'll also
push a SHA1 commit id so those wanting to know what they're running
(and be able to roll back if required) can use those instead.
Note that latest here is defined as "most recent master commit" not
"most recent released version", as the actual semantics of making latest
correct while still being able to build bugfixed releases of previous
versions is just ARGH. So we define it as "master" not "latest release".
