Drop python 3.5 support in eth module

[carver]

What is wrong?

Python 3.6 was released nearly 2 years ago. trinity is alpha software, which won't be stable for another good while. Python 3.5 doesn't support some sweet syntax like f-strings, native typing annotations, etc.

How can it be fixed

Let's drop py3.5 support from the eth module, and require >=3.6.

Objections?

[pipermerriam]

I believe we are held up by: Web3.py -> eth-tester -> py-evm eth module

[carver]

#1390 (comment)

Good point about web3.

Potentially web3 v4 can freeze py-evm at the current version, and updates to py-evm will only be available in web3 v5. (which I expect to be py3.6+)

[pipermerriam]

Our much ignored mailing list and /r/ethereum might be good places to solicit feedback (and prepare people).

[jacqueswww]

Good stuff! This is great from a vyper perspective - as vyper also requires 3.6.

[pacrob]

dropped in #1785