Docs: import reveal_type

[StefanVK]

In cheat sheet added import of reveal_type used in sample code

Also removed unused imports Union and Optional.

This is a tiny change but I was confused about where reveal_type was coming from and had to google it, so I figure it could be easier to understand for somebody else, too.

[StefanVK]

I actually misunderstood this. reveal_type as imported from typing also exists and does print the runtime type.
What's meant here is adding reveal_type(x) to the code, running mypy on it and mypy will print the type it tracked for the variable. mypy does not require import reveal_type from typing and adding the import may further confuse the point.

So I'm dropping this pull request.