Add Python 3.14 to the testing

[cclauss]

https://www.python.org/downloads/release/python-3140/

PyPy3.11: https://pypy.org/download.html

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Three failing tests because zstd was added to the Python >= 3.14 Standard Library

• PEP 784: A new module compression.zstd providing support for the Zstandard compression algorithm.

{'accept-encoding': 'gzip, deflate, zstd'} != {'accept-encoding': 'gzip, deflate'}

FAILED tests/test_item.py::test_upload - AssertionError: assert {'accept': '*...
FAILED tests/test_item.py::test_upload_metadata - AssertionError: assert {'ac...
FAILED tests/test_item.py::test_upload_queue_derive - AssertionError: assert ...