Declare Python 3.15 support

openstack-tox-py315 job is now voting in the -next job template.

Change-Id: I998cd164155155b6ca9f1421a3dd79ec4224302d
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",
 ]