🔧 Add free-threaded Python to tox conf

Author: veit

docs/test/tox.rst

@@ -165,7 +165,7 @@ Python versions:
    :emphasize-lines: 2, 4
 
    [tox]
-   envlist = py39, py310, py311, py312, py313
+   envlist = py3{9,10,11,12,13,13t,14,14t}
    isolated_build = True
    skip_missing_interpreters = True
 
@@ -253,7 +253,7 @@ extend commands to ``pytest --cov=items``:
    :emphasize-lines: 12-
 
    [tox]
-   envlist = py3{9,10,11,12,13}
+   envlist = py3{9,10,11,12,13,13t,14,14t}
    isolated_build = True
    skip_missing_interpreters = True
 
@@ -345,7 +345,7 @@ We can also call individual tests with tox by making another change so that
    envlist =
        pre-commit
        docs
-       py3{9,10,11,12,13}
+       py3{9,10,11,12,13,13t,14,14t}
        coverage-report
    isolated_build = True
    skip_missing_interpreters = True