build: enable free-threaded wheels in cibuildwheel

cibuildwheel does not build free-threaded CPython wheels by default, even
when the build pattern matches cp*. As a result, users on 3.13t/3.14t/3.15t
installed pure-Python wheels with no native extension and saw:

    Failed to load instrument hooks library: cannot import name
    'dist_instrument_hooks' from 'pytest_codspeed.instruments.hooks'

Opt in via `enable = ["cpython-freethreading"]` so the cp31{3,4,5}t wheels
ship the compiled dist_instrument_hooks extension.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: art049

pyproject.toml

@@ -103,6 +103,7 @@ push = false
 
 [tool.cibuildwheel]
 build = "cp*{manylinux,macosx}*"
+enable = ["cpython-freethreading"]
 test-extras = ["build", "test", "compat"]
 test-command = "pytest -v --ignore={project}/tests/benchmarks {project}/tests"