build: select free-threading builds with tox

Author: nedbat

tox.ini

@@ -4,7 +4,7 @@
 [tox]
 # When changing this list, be sure to check the [gh] list below.
 # PYVERSIONS
-envlist = py3{9,10,11,12,13,14}, pypy3, doc, lint, mypy
+envlist = py3{9,10,11,12,13,14}, py3{13,14}t, pypy3, doc, lint, mypy
 skip_missing_interpreters = {env:COVERAGE_SKIP_MISSING_INTERPRETERS:True}
 toxworkdir = {env:TOXWORKDIR:.tox}
 
@@ -54,6 +54,12 @@ commands =
 
     py3{12,13,14},anypy: python igor.py test_with_core sysmon {posargs}
 
+[testenv:py313t]
+basepython = python3.13t
+
+[testenv:py314t]
+basepython = python3.14t
+
 [testenv:anypy]
 # $set_env.py: COVERAGE_ANYPY - The custom Python for "tox -e anypy"
 # For running against my own builds of CPython, or any other specific Python.
@@ -128,5 +134,7 @@ python =
     3.11 = py311
     3.12 = py312
     3.13 = py313
+    3.13t = py313t
     3.14 = py314
+    3.14t = py314t
     pypy-3 = pypy3