Test with Python 3.15 alpha releases (#13654)

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 4.2.0
-envlist = py{311,312,313,314}
+envlist = py{311,312,313,314,315}
 
 [testenv]
 usedevelop = True
@@ -19,7 +19,7 @@ passenv =
     BUILDER
     READTHEDOCS
 description =
-    py{311,312,313,314}: Run unit tests against {envname}.
+    py{311,312,313,314,315}: Run unit tests against {envname}.
 dependency_groups =
     test
 setenv =