refactor(Session.new_window): PEP 3102 keyword-only arguments

Author: tony

src/libtmux/session.py

@@ -537,6 +537,7 @@ def rename_session(self, new_name: str) -> "Session":
     def new_window(
         self,
         window_name: t.Optional[str] = None,
+        /,
         start_directory: None = None,
         attach: bool = False,
         window_index: str = "",