Remove dependency to stable_baseline3

[verdimrc]

Propose to remove stable_baseline3 from requirements.txt, and move the affected implementations out from a2rl to the examples.

stable_baseline3 depends on gym-0.21 which conflicts to the newer version that a2rl requires. The old gym version doesn't come with complete type checks, and still with deprecated or removed gym APIs.

Currently, a2rl requires stable_baseline3 at these places:

• in data_generator_gym() which is used only in data_properties.ipynb.
• in the docstring of WhatifWrapper.

In all cases above, the examples still work (fortunately) with a newer gym.

[UPDATE] Alternatively, investigate whether it's possible to switch to mushroom-rl. It supports newer gym, but due to Cython requirement, need to check how easy is installation on ARM and DL1 instances.