👷 ci: test the free-threaded 3.15 build

Author: gaborbernat

.github/workflows/check.yaml

@@ -20,6 +20,7 @@ jobs:
       matrix:
         env:
           - "3.15"
+          - "3.15t"
           - "3.14"
           - "3.13"
           - "3.12"

tox.toml

@@ -1,5 +1,5 @@
 requires = ["tox>=4.38", "tox-uv>=1.29"]
-env_list = ["fix", "3.15", "3.14", "3.13", "3.12", "3.11", "3.10", "3.9", "3.8", "type-3.8", "type-3.14", "docs", "pkg_meta"]
+env_list = ["fix", "3.15", "3.15t", "3.14", "3.13", "3.12", "3.11", "3.10", "3.9", "3.8", "type-3.8", "type-3.14", "docs", "pkg_meta"]
 skip_missing_interpreters = true
 
 [env_run_base]