python 3.8 support

[grant-olson]

We should support python 3.8 or if that proves too difficult we should have a friendly error message instead of a big ugly traceback in the futures code.

[ikkez]

Hi there. The SDK actually run fine on python3.8.. it's just that the futures in 3.8 slightly changed the error handling. The problem with the "big ugly traceback" is that the connection is closed / destoried before all the listeners were shutdown. That was no problem in 3.7 and before and just failed silently. I'm not that deep into python yet, but in the SDK fork I did some time ago, I just workaround that's bug with a simple little timeout, see this lines:

ikkez@103fda2

That seems to work just fine, but there's probably a better, more correct way to solve that.