Declare Python 3.15 support

It's now voting in the -next job template.

Change-Id: Ibf02073c5250c92716712355b1ab3497382141e0
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

pyproject.toml

@@ -23,6 +23,7 @@ classifiers = [
   "Programming Language :: Python :: 3.12",
   "Programming Language :: Python :: 3.13",
   "Programming Language :: Python :: 3.14",
+  "Programming Language :: Python :: 3.15",
   "Topic :: Software Development :: Libraries",
   "Topic :: System :: Distributed Computing",
 ]