Add Python 3.13 to list of allowed Python versions (#11411)

## Summary

I believe we're already "Python 3.13-ready"? The main Ruff-impacting
change I see in https://docs.python.org/3.13/whatsnew/3.13.html is [PEP
696](https://peps.python.org/pep-0696/) which Jelle added in
https://github.com/astral-sh/ruff/pull/11120.

crates/ruff_python_formatter/src/options.rs

@@ -465,6 +465,7 @@ pub enum PythonVersion {
     Py310,
     Py311,
     Py312,
+    Py313,
 }
 
 impl PythonVersion {