Timezone naive expected

Due to upcoming changes in Numpy deprecating timezone-aware handling by numpy.datetime64, and the numerous essential libraries which depend on Numpy,
we have defaulted to timezone-naive.

The .forceutc() function still allows coercing to UTC if desired, but this is no longer used internally as it used to be.

Also CI now uses flake8 and mypy type checking. As usual, type checking helped uncover some corner cases.