ENH: add type hints

[venaturum]

sources:
https://docs.python.org/3/library/typing.html
https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html
https://realpython.com/lessons/type-hinting/

https://github.com/staircase-dev/staircase/blob/master/staircase/_typing.py has been created for custom types (see https://github.com/pandas-dev/pandas/blob/master/pandas/_typing.py for inspiration)