feat: use future annotations for same class type

[dluo96]

Is your feature request related to a problem? Please describe

This issue is about cleaning the type hinting in the repository. Currently, when a method (inside a given class) returns an object whose type is the class itself (e.g. in Environment), the return type is given with quotes:

class Environment:

    ...

    def unwrapped(self) -> "Environment":
        ...

According to this thread, this is needed for Python<3.7, but can be removed with future annotations from Python 3.7+.

Describe the solution you'd like

Since Jumanji supports Python>=3.8, we could get rid of these quotes throughout the repository and use future annotations instead.

[dantp-ai]

I can take this as a first issue :).