Add python 3.7 to CI tests

[dylanjw]

What was wrong?

python 3.7 is out, and we should include it in our testing.

How can it be fixed?

add python 3.7 to our tox.ini & circleci config

[veox]

For ref: ethereum/py-evm#970 - same-topic tracking issue in py-evm; with some initial findings - haven't revisited in 2 weeks.

[voith]

python3.7 support in websockets was added in v6.0. So websocket version has to be bumped in web3 to 6.0.

[voith]

In #973, someone has already tested HTTPProvider with python3.7 and it seems functional. websockets might be the only library that might need a version upgrade.

[voith]

I tried testing python3.7 with my fork today, but the build is failing. https://circleci.com/gh/voith/web3.py/3206
Looks like we'll have to wait for py-evm to work with python3.7. Curently, plylevel gives the following error:

  plyvel/_plyvel.cpp:547:24: fatal error: leveldb/db.h: No such file or directory
   #include "leveldb/db.h"
                          ^
  compilation terminated.
  error: command 'gcc' failed with exit status 1

Looking at ethereum/py-evm#1109, it seems like it can fixed by bumping the plyvel version.

[veox]

py-evm should now be good with Python 3.7.

@voith - seems you've already started working on this; could you craft a PR?..

[voith]

@veox Thanks for informing. I was waiting for py-evm to be released, so that I can make the corresponding change in eth-tester and web3

[voith]

waiting on ethereum/eth-tester#120

[voith]

all blockers removed. #996 should close this issue.