Add type annotations where useful (constructors, methods, functions, etc.)

[MatthewCaseres]

Is your feature request related to a problem? Please describe.
When typing out arguments I want to see what I should be passing in.

Is your feature request at odds with the scope of the package?
dont think so

Describe the solution you'd like
Put type annotations so that there are minimal "Any" in this screenshot

Describe alternatives you've considered
Letting the code remain as it is.

Additional context

Currently there is no minimal version of Python in setup.py. Type annotations are from 3.5, so it might be worthwhile to add this as requirement.

Ideally the type annotation for origin_grain is Literal['Y', 'S', 'Q', 'M'], and typing.Literal is from 3.8, and 3.8 is also the earliest version that is not end of life - https://devguide.python.org/versions/. So I want to add an earliest Python version to setup.py and make it 3.8.

[genedan]

I'll take this one on.