pyproject.toml: Add trove classifier for Python 3.12

https://github.com/pypa/trove-classifiers/blob/main/README.md

pyproject.toml

@@ -17,6 +17,7 @@ classifiers = [
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: Implementation :: CPython",
     "Programming Language :: Python :: Implementation :: PyPy",
     "Operating System :: POSIX",