DOC: Fix Python version specification

Python 3.9 was specified twice, so I removed one.
Python 3.12 was not specified but is now. This is possible as the
RCs are already out and I doubt that it will cause any breaking
changes for Python only projects.

Author: Kai-Striega

pyproject.toml

@@ -22,9 +22,9 @@ classifiers = [
     "Programming Language :: Python",
     "Programming Language :: Python :: 3",
     "Programming Language :: Python :: 3.9",
-    "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3 :: Only",
     "Topic :: Software Development",
     "Topic :: Office/Business :: Financial :: Accounting",