Make use of @final, Final and Literal - #3753

Closes #3735

• Mostly changes ClassVar to Final for all constants
• Adds some limited usage of the @final decorator for Base{UpdateProcessor, Request}
• Uses Literal for the http_version argument of HTTPXRequest. Otherwise, Literal is not used due to Literal of enum values python/typing#781

If there are more places, where @final or Literal are useful, please do point them out :)