Add testing for Python 3.7

[carver]

What is wrong?

Python 3.7 stable has been released: https://pythoninsider.blogspot.com/2018/06/python-3.html
We are not currently running any tests against it.

How can it be fixed

Start running the core tests against py3.7 in addition to the current python envs.

[veox]

CircleCI does not yet provide machine images with pre-installed Python 3.7 stable release - only beta 5 and the release candidate.

Watch docs or docker hub (linked from docs) for when those become available.

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EDIT: For a comparison between beta 5 and the RC - see Python's change log.

[veox]

Some dependencies might not yet support Python 3.7: see failed job:

idna-ssl requires Python '<3.7.0' but the running Python is 3.7.0

EDIT: Arch Linux users are very likely to get hit with this first. See package page for when it enters testing/extra repos.

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For ref, I used this branch of mine. It requests the python3.7-rc image from CircleCI, but manual inspection via SSH shows the machine had beta 5 installed:

circleci@e5fdac382095:~$ python --version
Python 3.7.0b5

EDIT: I'd say better wait for an image with 3.7 stable to avoid such shenanigans.

[carver]

The first exploratory steps of seeing what else is broken besides idna-ssl can be started. Totally agree that we should wait until a stable 3.7 image is released before it goes into CI.

[veox]

Summary: still waiting.

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Images with Python 3.7.0 (the stable version) have become available.

I've made a new branch and directed it to use this new machine image, but there are still dependency problems.

The plyvel Python package does not have a pre-built wheel available for Python 3.7; so the tox run attempts to build the package itself; the system packages libleveldb-dev and libgmp-dev are not installed on the (Debian 9, "Stretch") machine, so the build fails.

After SSH'ing in and installing them, the build still fails, with same error as in wbolster/plyvel#62, i.e. because building plyvel requires leveldb header version at least 1.20, whereas Stretch has 1.18.

Possible approaches:

1. wait for plyvel to get a py3.7 wheel, or help it happen; alternatively,
2. enable the Debian testing repo ("Buster"), which has libleveldb-dev 1.20.

(2) can get very brittle, so IMO best avoid it.

[pipermerriam]

We can probably open a PR to plyvel to enable testing against 3.7. Once that is passing I doubt it would take too much coaxing to get them to push out a new release with 3.7 support.

Alternatively, we've wanted to explore RocksDB (but I don't know if whatever python library sits on top of that supports 3.7 either)

[hairychris]

There is an issue with the Travis CI server and Python 3.7 but wheels can come before that
wbolster/plyvel#79

[dylanjw]

It looks like plyvel 1.05 has been pushed with a py3.7 wheel

[veox]

Issue no longer blocked: I've had a successful CI job for core tests with Python 3.7.

Currently cleaning up that branch, will submit a PR.