What should our minimum python 3 version be?

As CoreTools moves to be python 3 first and deprecates python 2.7 over the next year, there is an important question to answer in terms of the minimum python 3 version that we should support. This issue exists to document that discussion.

It seems like the decision is down to requiring Python 3.5+ or Python 3.6+. Support python 3.4 is out because it does not properly allow for `async/await` to be used inside `asyncio` event loop.

In terms of the supporting Python 3.5+ my biggest outstanding question would be understanding if there are use cases, especially in the embedded context (that tends to lag behind) that would still have a python 3.5 interpreter standard.

@dkarchmer @mattrunchey