Re-enable free-threaded Python (3.13t/3.14t) CI/wheels (currently skipped due to segfaults)

[phaarnes]

Summary

Free-threaded Python builds (e.g., 3.13t/3.14t) are currently skipped in CI/wheel builds because tests segfault (seen in repr paths for Cython extension). We should track what’s needed to safely re-enable these builds.

Context:

• PR Fix tests and workflows #1571 added CIBW_SKIP: "*t-*" to avoid segfaults on free-threaded Python.
• Reported as affecting at least Linux aarch64 and macOS x86_64 in CI.

Current status

• Free-threaded Python builds are disabled/skipped in cibuildwheel via CIBW_SKIP: "*t-*".

Re-enabling free-threaded builds

To re-enable *t-* Python builds:

• Identify and reproduce the segfault under free-threaded CPython.
• Determine whether the root cause is in pyproj, Cython, or an upstream dependency.
• Apply a fix or upstream workaround.
• Re-enable *t-* builds in cibuildwheel once tests pass reliably.

References

• PR: Fix tests and workflows #1571 (introduced skip)
• Issue: CI workflow failures: sdist check, Windows build, macOS build time, and free-threaded Python #1570 (CI/workflow failures bundle; includes the free-threaded skip rationale)