Python 3.8 type annotations

[MischaPanch]

Hi, thank you for the great library! I started using it today and noticed that quite some parts of the code rely on string interfaces. For example. DDPGPolicy.forward accepts model as a string and proceeds to using __getattr__. There is no type and no documentation on the allowed inputs, which here are "actor" and "actor_old".

Now my question: would you be interested in dropping python 3.6 support in favor of 3.8 or higher? It's been a while since that's been out and it has a much richer type system. For example, here one could annotate with Literal["actor", "actor_old"], which would significantly improve the readability as well as type safety.

I would be happy to open a PR then

[MischaPanch]

Btw, gymnasium only supports python >=3.8, so there is no real point maintaining support for lower versions in my opinion

[Trinkle23897]

#910