Pass static typing tests

[chrislit]

Add static type hinting & test with mypy. This issue will get moved from release to release until each task is complete (which might never occur).

Tasks:

• complete mypy abydos with no errors
• remove calls to cast (they're not free...)
• add type hints to every function/method (enable disallow_incomplete_defs = True & disallow_untyped_defs = True in [mypy] of setup.cfg)
• clean out Union types (consider making simpler non-Unions)
• eliminate # type: ignore
• transition variable type hints to Py3.7+ style (after removing Py3.6 support)

[chrislit]

I may do a bit of this before releasing 0.6, but 3.6-support probably will stay until 0.6's release, so the final task can't be done yet.