Software is no longer compatible with Python 3.6 (PYNQ OS 2.6)

A user told me that #264 breaks compatibility with Python 3.6; they get an error from `from __future__ import annotations`. Not a big deal since this only affects PYNQ OS 2.6, which is only being used by very early adopters of QICK.

We might fix this when we have time, or we might decide to mark this as don't-fix, but for now this issue will serve as a reminder.