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

Signed-off-by: Christopher May-Townsend <(chris@maytownsend.co.uk)>

Python 3.7 is apparently faster than 3.6, and should be mature enough.

Including workers!

* 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

Switch the matrixdotorg/synapse:latest Docker image to use python 3

Give the image the right tag, so that we can push it.

... otherwise it hangs

the docker image doesn't have wget

Docker containers spin up faster than entire VMs.

The additional sha1 tagged builds in docker hub are messy,
so instead tag the build with the SHA1 version.

Note the regex must match the complete string anyway,
so the leading ^ was useless.

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".

* changelog

* reduce circleci config

* plus a handy script

* fix regex

* fix

* changelog