Use compression.zstd (PEP-784)

[Rogdham]

Python 3.14 introduces support for Zstandard in the standard library via the compression.zstd module (PEP-784).

This PR removes the zstdandard extra dependency in favor of the native support in Python 3.14+. For older Python versions, the backport is used.

It also has the advantage of supporting free threading.

Related: Test on Python 3.14 #2361

Full disclosure: I'm the author and maintainer of backports.zstd, and the maintainer of pyzstd (which code was used as a base for the integration into Python). I also helped with PEP-784 and its integration into CPython.

[Copilot]

Pull Request Overview

This PR migrates from the third-party zstandard package to Python's native compression.zstd module (introduced in Python 3.14 via PEP-784), with backports.zstd as a fallback for older Python versions.

Key Changes:

• Replaced zstandard==0.25.0 dependency with backports.zstd==1.0.0 for Python < 3.14
• Updated import logic to use native compression.zstd on Python 3.14+ or backports.zstd otherwise
• Simplified compression/decompression by using built-in compress/decompress functions instead of wrapper functions

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
requirements/extras/zstd.txt	Updated dependency from zstandard to backports.zstd with Python version constraint
kombu/compression.py	Changed imports to use native or backport zstd, replaced custom wrapper functions with direct API calls
t/unit/test_compression.py	Updated test fixtures to check for zstd availability at module level instead of per-test imports

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[Rogdham]

t/unit/test_compression.py Updated test fixtures to check for zstd availability at module level instead of per-test imports

I reverted this to fix the lint error I had. Now linting is green!