[yt-dev] yt-4.0 branch and python 3-only features

Hi folks, Is it now safe to use Py3-only features? If so, are we setting Python 3.6 to be the minimum version? YTEP-0033 doesn't note specifically. The specific thing I want to use (using native metaclasses instead of @add_metaclass) isn't Python 3.6 only, but I thought it was as good a time as any to ask. -Matt