Declare Python 3.15 support

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

Change-Id: I9e44f5903c5c9abe2557a36578750006f94bf1f2
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",
     "Programming Language :: Python :: 3 :: Only",
     "Programming Language :: Python :: Implementation :: CPython",
 ]