Make use of @final, Final and Literal

[Bibo-Joshi]

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 :)

[lemontree210]

I don't have the required in-depth knowledge to suggest more cases of @final that may have been missed