update mypy to use --no-implicit-optional arg

[kristapratico]

This mypy option will enforce using typing.Optional when a default argument of None is stated:

def func(a: int = None):
    return a

main.py:36: error: Incompatible default for argument "a" (default has type "None", argument has type "int")

[kristapratico]

The pyright equivalent is strictParameterNoneValue. https://github.com/microsoft/pyright/blob/main/docs/configuration.md

[kristapratico]

We get this for free with pyright and we'll get this in mypy when we bump our currently pinned version (TBD) because this option is now enabled by default.