New rule: prefer x: Final = 42 over x: Final[Literal[42]]

[tomasr8]

closes #468

Here it goes :)

There is an interaction between this new rule (Y064) and Y061 (which warns for Literal[None]) where it's not entirely clear to me what to do. For example, in this case:

x: Final[Literal[None]]

you'd get both Y064 and Y061. I think in this case turning it into x: Final = None is probably nicer so I decided to not emit Y061 if Y064 would also be emitted.

On the other hand, this seems like a pretty unusual case (no examples in typeshed) so maybe it's not needed?

[github-actions]

⚠ Flake8 diff showing the effect of this PR on typeshed:

> ./stubs/croniter/croniter/croniter.pyi:39:5: Y064 Use "MONTHS_IN_YEAR: Final = 12" instead of "MONTHS_IN_YEAR: Final[Literal[12]]"
> ./stubs/pika/pika/compat.pyi:16:1: Y064 Use "PY2: Final = False" instead of "PY2: Final[Literal[False]]"
> ./stubs/pika/pika/compat.pyi:17:1: Y064 Use "PY3: Final = True" instead of "PY3: Final[Literal[True]]"
> ./stubs/pika/pika/compat.pyi:48:1: Y064 Use "EINTR_IS_EXPOSED: Final = False" instead of "EINTR_IS_EXPOSED: Final[Literal[False]]"

[srittau]

Thanks!