ci: use certain OS version instead of latest

[peaceiris]

• [x] ci: Change macos-latest to macos-10.15
• [x] ci: Change windows-latest to windows-2019
• [x] ci: Add macos-11.0 (macOS Big Sur 11.0)
• [ ] docs: Add macos-11.0 (macOS Big Sur 11.0)
• [ ] ci: Add deploy step for macos-11.0 (macOS Big Sur 11.0)

https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions#jobsjob_idruns-on

[codecov[bot]]

Codecov Report

Merging #530 into main will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #530   +/-   ##
=======================================
  Coverage   61.17%   61.17%           
=======================================
  Files           4        4           
  Lines         273      273           
  Branches       48       48           
=======================================
  Hits          167      167           
  Misses        105      105           
  Partials        1        1           

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