Convert Sphinx docstrings to mypy

[amorphic]

Sphinx now supports Python 3 type annotations via an extension.

Update all opengb functions + methods with type annotations and then remove the excess type lines in the docstrings to reduce line count. This won't introduce new features or fix bugs but will make the codebase easier to maintain.

Note: As opengb is Py3 only we can adopt this without breaking backwards compatibility.