upgrade gym version to >=0.21, fix related CI and update examples/atari

[gogoduan]

• I have marked all applicable categories:
  • exception-raising fix
  • algorithm implementation fix
  • documentation modification
  • new feature
• I have reformatted the code using make format (required)
• I have checked the code using make commit-checks (required)
• If applicable, I have mentioned the relevant/related issue(s)
• If applicable, I have listed every items in this Pull Request below

[codecov-commenter]

Codecov Report

Merging #534 (25980d6) into master (c7e2e56) will decrease coverage by 0.40%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master     #534      +/-   ##
==========================================
- Coverage   94.43%   94.03%   -0.41%     
==========================================
  Files          63       63              
  Lines        4298     4306       +8     
==========================================
- Hits         4059     4049      -10     
- Misses        239      257      +18     

Flag	Coverage Δ
unittests	94.03% <54.54%> (-0.41%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tianshou/data/collector.py	94.16% <50.00%> (-1.42%)	⬇️
tianshou/__init__.py	100.00% <100.00%> (ø)
tianshou/policy/multiagent/mapolicy.py	85.71% <0.00%> (-11.91%)	⬇️
tianshou/policy/modelfree/trpo.py	88.52% <0.00%> (-4.92%)	⬇️
tianshou/env/pettingzoo_env.py	91.30% <0.00%> (-4.35%)	⬇️
tianshou/policy/modelfree/npg.py	98.85% <0.00%> (+1.14%)	⬆️

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