Create `Unit` type alias for `None`

[sobolevn]

Sometimes it might be helpful to make a differce between -> None and -> Unit.

We need to investigate how it is possible.

Examples:

def some() -> Unit:
    print('Just prints')

class Some():
    def __init__(self) -> None: ...

[sobolevn]

Or, we can use () as a unit type python/mypy#4211

[sobolevn]

This will be very hard to read for newcovers. Let's leave it as good-old None.

By the way, () won't work as a unit value. You would have to explicitly return it everywhere. Not a great idea.