Declare Python 3.15 support

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

Change-Id: I827b345169b0790fad49dfa943b8f724ccd85853
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

pyproject.toml

@@ -25,6 +25,7 @@ classifiers = [
   "Programming Language :: Python :: 3.12",
   "Programming Language :: Python :: 3.13",
   "Programming Language :: Python :: 3.14",
+  "Programming Language :: Python :: 3.15",
   "Programming Language :: Python :: 3 :: Only",
   "Programming Language :: Python :: Implementation :: CPython",
 ]