Add Pytest

[rodrigodelazcano]

Description

This PR adds the following:

• Remove compatibility with python 3.6 due to python foundation not supporting it any more and dependency issues when using entrypoint plugins (https://github.com/Farama-Foundation/gym-minigrid/pull/205#issue-1326438686)
• Fully implemented the Renderer Gym API with render_mode. This won't break other libraries since the old rendering API can still be used.
• Moved benchmark.py and manual_control.py to gym_minigrid\ directory.
• Add Pytest to the Github CI. Pytests are organized in two files tests\test_envs.py and tests\test_wrappers.py. These have a combination of tests previously found in run_tests.py and test_interactive_mode.py, the env_checker test from Gym, and new tests. The division of the pytest functions are as follow:

1. From old run_tests.py and test_interactive_mode.py: test_envs.py:test_agent_sees_method(), test_envs.py:old_run_test(), test_envs.py:test_interactive_mode(), test_wrappers.py:test_dict_observation_space_wrapper(), test_wrappers.py:test_main_wrappers(), test_wrappers.py:test_observation_space_wrappers(), test_wrappers.py:test_agent_sees_method()

2. From Gym: test_envs.py:test_env(), test_envs.py:test_env_determinism_rollout(), test_envs.py:test_render_modes()

3. New tests: test_wrappers.py:test_reseed_wrapper(), test_wrappers.py:test_state_bonus_wrapper(), test_wrappers.py:test_action_bonus_wrapper()

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Screenshots

Please attach before and after screenshots of the change if applicable.

Checklist:

• [x] I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

[rodrigodelazcano]

I updated the PR with the changes. Also fixed the deprecated None in pytest.warn()

[pseudo-rnd-thoughts]

LGTM