Declare Python 3.15 support

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

Change-Id: I3300a1caadab1f6a8d85acb3b3a92faea3907304
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

pyproject.toml

@@ -22,6 +22,7 @@ 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",
     "Typing :: Typed",