Use random seed in reset

[javiarrobas]

Currently, random seeds are set using the random package where needed. See for example here.
This issue is to update the repository to latest practice by using a seed argument in the reset method.
In fact, with the recent update of this repository to Gym v26 we have accommodated the reset method to comply with the latest interface that already accepts a seed as an argument in reset (see here). However, this new argument is omitted in practice since it is not yet used in the examples nor there is functionality that effectively implements the passed seed. This issue should change that.

[javiarrobas]

See comment message in e533cea for the vectorized environment.