Pymap3d

For better internal quality, modernize type annotations in accord with Numpy 1.20 final release.
This requires [`__future__.annotations`](https://docs.python.org/3/library/__future__.html), 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](https://github.com/geospace-code/pymap3d/tree/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.

Numpy 1.20 adds optional type hinting in the numpy.typing module. ArrayLike is a key type we feel appropriate for PyMap3D. PyMap3D will continue to work without Numpy or older versions of Numpy.

We add optional type hinting using numpy.typing.ArrayLike

* remove extraneous syntax from tests
* add codecov

The ECI calculation error was substantial for some applications due to only considering Earth rotation. Now ECI calculations use AstroPy for better accuracy (< 1% error) considering more factors in the ECI transformations.
Fixes 33

Added `geodetic2eci`

Added more ECI self-tests

Temporarily pinned CI to Python 3.8 to avoid the usual issues with brand new Python releases not immediately having PyPi wheels for Astropy. We still also test Python 3.5.