Backward compatibility with version 2.10.0

[TrickSonix]

Hello. I am using pyrate-limiter in my project as supporting module for rps-control. My project was made with version 2.10.0 of pyrate-limiter and i was using class RequestRate. But in version 3.1.0 this calss was removed. And whenever Me, or anybody, who used my project first time and updating pyrate-limiter, all my projcet no longer runs via import errors.

There is no problem to rework some code in my project, but i think that general backward compatibility will be a good idea. Just mark those function/classes that will be deprecated in future. So everybody will be aware of it, and fix code where it needed.