gym.spaces.Dict inherits from collections.abc.Mapping (#2446)
gym.spaces.Dict inherits from collections.abc.Mapping (#2446)

* gym.spaces.Dict inherits from collections.abc.Mapping

It would be very convenient to have `gym.spaces.Dict` inheriting from `collections.abc.Mapping` so that it can be used in conjunction with [dmtree](https://github.com/deepmind/tree) to perform operations on complex spaces conveniently. It also simplifies the implementation which is good.

I just don't like the fact that the `contains` method behavior is not consistent with what it should be doing if it was a proper mapping...

* Update dict.py

* Update dict.py
Farama-Foundation/Gymnasium,Farama-Foundation/Gymnasium