[emmatyping]

This is part 2 (but parallel to part 1) for implementing PEP 784. This is probably the meat of the implementation making up ~half of the LOC in the overall diff.

This change:

• Adds the pyzstd license, since this is the first code introduced based on pyzstd
• Adds the code for the _zstd module in Modules/_zstd. Tests will be included when the Python compression.zstd module is added. If people think the tests should be included, I can merge the compression.zstd changes into the PR, but I wanted to separate them to make the PRs a bit more manageable.
• Adds unix build changes (Windows requires changes in cpython-source-deps and will be added in a follow-up PR)

I added skip news as I'd like to write a holistic NEWS/What's New entry once the entire implementation has landed. If people think each PR should have NEWS I can write something up.

TODO before merge:

• try running tests on top of this branch
• format PEP 7 style
• run buildbots on this PR
• verify _zstd gets installed

• Issue: Implement PEP 784 - Adding Zstandard to the Python standard library #132983

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📚 Documentation preview 📚: https://cpython-previews--133027.org.readthedocs.build/