CI: Don't run tests on all Python versions every PR #90
Description
Same as TeamEpochGithub/epochlib#202. On PRs, only run tests for 3.10. On main, run tests for 3.10, 3.11, and 3.12.
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Same as TeamEpochGithub/epochlib#202. On PRs, only run tests for 3.10. On main, run tests for 3.10, 3.11, and 3.12.