Fix readthedocs build failure

[Trinkle23897]

No description provided.

[codecov-commenter]

Codecov Report

Merging #803 (e3cf4e6) into master (6c6c872) will decrease coverage by 0.04%.
The diff coverage is 50.00%.

❗ Current head e3cf4e6 differs from pull request most recent head ed30365. Consider uploading reports for the commit ed30365 to get more accurate results

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@@            Coverage Diff             @@
##           master     #803      +/-   ##
==========================================
- Coverage   91.15%   91.12%   -0.04%     
==========================================
  Files          73       73              
  Lines        5079     5082       +3     
==========================================
+ Hits         4630     4631       +1     
- Misses        449      451       +2     

Flag	Coverage Δ
unittests	91.12% <50.00%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
tianshou/env/utils.py	73.68% <50.00%> (-7.57%)	⬇️

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