Combine coverage from symlinked API files

[ajfriend]

Figure out how to combine coverage from the multiple h3.api.*.__init__.py files (since they are the same) instead of excluding 3 out of 4 of them to get coverage to pass.

I couldn't figure this out in my first attempt in #334

See: https://coverage.readthedocs.io/en/latest/cmd.html#re-mapping-paths

This might be easier to figure out using the raw coverage commands rather than through pytest-cov.