Fix type annotations of buffers

[araffin]

Description

Fixes replay buffer type annotation, starting to fade out pytype too (the idea would be to have mypy only at the end, faster and stricter).

closes #112 too

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[qgallouedec]

I'll look into this tomorrow :)

[qgallouedec]

The ideal for buffers would be to use a generic annotation, but for the sake of simplicity, I think it's better to keep it this way.

[araffin]

The ideal for buffers would be to use a generic annotation, but for the sake of simplicity, I think it's better to keep it this way.

I actually tried but reverted back.... there was no simple way to tell mypy that the type in would be the same out with all the inheritance.
We would need some inheritance à la rust, where you inherit features and not a complete class...