Adopt Py3.8+ features: New Typing features

[Bibo-Joshi]

Python 3.8 adds the

• @final decorator
• Final type
• Literal type

to the typing module. This allows to make some of our type hints more precise.
We should review our code bases and

• use the @final decorator for classes and methods that should not be subclassed/overridden, e.g. in interface classes like Base{Persistence, Handler, UpdateProcessor, Request}
• Use Final rather than ClassVar for class attributes that are not supposed to be overridden, e.g. Chat.PRIVATE. Also check if there are other attributes that should be considered final
• Use Literal where it makes sense. These may be very few places, considering that it does not play well in combination with enums (Literal of enum values python/typing#781)

A PR in this direction is very welcome. As long as #3728 is open, it should be based on the drop-py-37 branch. If you would like to send a PR, please be sure to read our contribution guide and leave a short comment so that we can assign you.