feat: add option to opt-in free-threaded builds

unit_test/build_selector_test.py

@@ -50,6 +50,7 @@ def test_build_filter_pre():
     assert build_selector("cp313-manylinux_x86_64")
     assert build_selector("cp37-win_amd64")
     assert build_selector("cp313-win_amd64")
+    assert not build_selector("cp313t-manylinux_x86_64")
 
 
 def test_skip():
@@ -144,6 +145,14 @@ def test_build_limited_python_patch():
     assert build_selector("cp37-manylinux_x86_64")
 
 
+def test_build_free_threaded_python():
+    build_selector = BuildSelector(
+        build_config="*", skip_config="", prerelease_pythons=True, free_threaded_support=True
+    )
+
+    assert build_selector("cp313t-manylinux_x86_64")
+
+
 def test_testing_selector():
     # local import to avoid pytest trying to collect this as a test class!
     from cibuildwheel.util import TestSelector