Add license information to pyproject.toml

[ecederstrand]

Add license specifiers according to https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-and-license-files

This allows automatic license inspection tools like pip-licenses to correctly categorize this package.

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Fixes #16603 🦕

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[gemini-code-assist]

Code Review

This pull request adds license metadata to the pyproject.toml file for the google-crc32c package. The reviewer suggests using the PEP 621 table format for the license field to ensure better compatibility with various tools and notes that such configuration updates should ideally be applied consistently across all packages in the repository.