Add type annotations for all definitions

[Cito]

We should add type annotations for all definitions except those in tests, and enforce this with a mypy.ini configuration like this:

[mypy]
python_version = 3.8
...
disallow_untyped_defs = True

[mypy-tests.*]
disallow_untyped_defs = False

[KingDarBoja]

Could you provide an example regarding type annotations for definitions that are missing? Cheers!

[Cito]

@KingDarBoja When you set disallow_untyped_defs = True, mypy will list all the missing definitions, like missing return type of a __str__ method, and missing type annotations for internal functions.

[Cito]

I have now added all the missing definitions, except for a few modules where it makes no sense or where I'm waiting for #99.