Add type hinting

[venaturum]

There are currently no type-hints in staircase.

Background/resources:
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)

Please add new issue for each individual file with typing added, and commit separately.

[PabloRuizCuevas]

Hi! I wouldn't definitely close this issue, as there are some type hints I wasn't able to figure out, or maybe let this close but we could do a new issue for mypy compliance. Whatever you think is best :)

[venaturum]

@PabloRuizCuevas no worries