Addresses Windows compatibility issues

[desertaxle]

• Adds os to matrix in python-test workflow to run tests on Windows
• Uses PurePosixPath to ensure the correct path separator is used when working with GCS blobs

Closes #116

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• References any related issue by including "Closes #" or "Closes ".
  • If no issue exists and your change is not a small fix, please create an issue first.
• Includes tests or only affects documentation.
• Passes pre-commit checks.
  • Run pre-commit install && pre-commit run --all locally for formatting and linting.
• Includes screenshots of documentation updates.
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• Summarizes PR's changes in CHANGELOG.md

[ahuang11]

Looks good!