MyPy PEP 585, 604 support

MyPy 0.800 added support for PEP 585 and 604, which bring the new concise Python 3.10 type annotation syntax to earlier Python versions. The new type annotation syntax works all supported Python versions, if each file using them has at the top:

from __future__ import annotations

Separately, Numpy 1.20 made the long-awaited Numpy type hinting a reality. As too often the case, I became aware of these changes by newly failing CI cases. I was able to update syntax fairly quickly to take benefit of these new type annotation features.