Declare Python 3.15 support

Python 3.15 unit test job is now made voting by the -next job template.

Change-Id: Iaa4877236aa479366c96737c32a5c380c93c0b2f
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

pyproject.toml

@@ -24,6 +24,7 @@ classifiers = [
   "Programming Language :: Python :: 3.12",
   "Programming Language :: Python :: 3.13",
   "Programming Language :: Python :: 3.14",
+  "Programming Language :: Python :: 3.15",
 ]
 
 [project.urls]