Python >= 3.7, modernize type annotations

For better internal quality, modernize type annotations in accord with Numpy 1.20 final release.
This requires __future__.annotations, which requires Python >= 3.7.
For many months, Python 3.5 use of pymap3d has been ≪ 1%
Python 3.6 use of pymap3d continues to drop as current Numpy requires Python >= 3.7 as well.

Python 3.5 and 3.6 users can continue to use pymap3d 2.4.x.
If we find any significant bugs, we plan to backport them to pymap3d 2.4.x as long as there is still significant pymap3d usage from Python < 3.7.