Fixes #3729

Summary/Motivation:

This adds Python 3.14 as a supported Python version. In addition to updating documentation, this PR needed to make a couple additional changes to the build system and codebase.

Changes proposed in this PR:

• Add Python 3.14 as a supported cinterpreter
• Update 3 tests to track changes in exception messages in Python 3.14
• Update pyproject.toml to core metadata 2.4 for compatibility with newer setuptools (under Python 3.14)
• Remove build system hacks to work around 3.13.5 on Windows.
• Remove linear-tree as a dependency on Python 3.14 so that we can remove the scikit-learn version pin for that interpreter (because older versions are not available for Python 3.14)
• Ensure that we always downgrade setuptools on Windows to work around bug in the APPSI build harness.

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