Rationale for this change

Closes #3322

This is mostly autogenerated by Claude to test the feasibility of 3.14t Looks like there are still dependent libraries without 3.14t support (bodo / ray / sql-postgres / hive-kerberos / datafusion)

Adds support for building and testing pyiceberg under free-threaded Python (3.14t).

Changes

Cython extension (decoder_fast.pyx, setup.py, pyproject.toml)

• Add # cython: freethreading_compatible=True directive to decoder_fast.pyx
• Detect Py_GIL_DISABLED at build time and set freethreading_compatible compiler directive in setup.py
• Bump Cython build requirement from >=3.0.0 to >=3.1.0 (required for freethreading_compatible)

Thread safety (singleton.py)

• Add double-checked locking to the Singleton metaclass to prevent races without the GIL

Build system (Makefile)

• Detect free-threaded Python via PYTHON=3.14t and automatically exclude extras that lack free-threaded wheels (bodo, ray, sql-postgres, hive-kerberos, datafusion)
• Skip corresponding test files (test_datafusion.py, test_upsert.py, kerberos tests)

CI workflows

• Add 3.14t to the lint-and-unit-test matrix in python-ci.yml
• Enable free-threaded wheel builds in pypi-build-artifacts.yml and svn-build-artifacts.yml (CIBW_FREE_THREADED_SUPPORT: "true", remove CIBW_SKIP: "cp3*t-*")

Are these changes tested?

Are there any user-facing changes?