Support Python 3.7

[smartcontracts]

Issue Type

[ ] bug report
[x] feature request

Current Behavior

Everything breaks when we use Python 3.7 as a result of a dependency on asyncio and some keyword conflicts. See issue #167 for more information.

Expected Behavior

Tests should work on Python 3.7.

Steps to Reproduce

1. Use Python 3.7
2. Try to test (make test)
3. See everything break.

• Full output of error:

  File "/Users/.../plasma-mvp/env/lib/python3.7/site-packages/websockets-4.0.1-py3.7-macosx-10.13-x86_64.egg/websockets/compatibility.py", line 15
  asyncio_ensure_future = asyncio.async           # Python < 3.5
                                      ^
  SyntaxError: invalid syntax

• Command that caused error: make test

Suggested Fix

Use Python 3.6 for now, try to find a fix for this (probably upgrade websockets?)

Motivation for Change

Lots of people use py3.7, we should support it.

[smartcontracts]

Seems like we should be able to fix this by bumping Web3.py to 4.5.0 (?). Testing now.