Python 3.14: PEP-784 compression.zstd

[Rogdham]

Second PR about PEP-784 in stdlib (first PR):

• Add type hints for _zstd
• Add type hints for compression.zstd

[Rogdham]

@emmatyping: if you want to give try this out, or simply comment on the use of Mapping[CompressionParameter, int] and Mapping[DecompressionParameter, int] as type hints for the options parameter. Any feedback would be appreciated!

[emmatyping]

Note that I typed the options as Mapping[CompressionParameter, int] / Mapping[DecompressionParameter, int], while a Mapping[int, int] is technically accepted.

I think we should type that it accepts Mapping[int, int] as well. I consider that a supported usage.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[Rogdham]

Now that CPython 3.14.0 beta 2 is released, the CI is green and the PR ready for review!