Pass tempfile_suffix argument to PromptSession.

This ensures that when the open_in_editor() method of the current buffer (a prompt_toolkit.buffer.Buffer) is called, the tempfile opened in the editor will always have a .py extension, to support editor syntax highlighting.

The open_input_in_editor(event) function associated with the f2 keybinding manually sets the tempfile_suffix property of the current buffer before calling its open_in_editor() method. The vi keybinding (v) and emacs mode keybinding (c-x c-e) are handled by prompt_toolkit directly, bypassing this function, so the tempfile created when they are used is a .txt file. Passing the tempfile_suffix argument to the PrompSession constructor ensures that .py extension is always used.

IPython/terminal/interactiveshell.py

@@ -322,6 +322,7 b' class TerminalInteractiveShell(InteractiveShell):'
                             mouse_support=self.mouse_support,
                             enable_open_in_editor=self.extra_open_editor_shortcuts,
                             color_depth=self.color_depth,
+                            tempfile_suffix=".py",
                             **self._extra_prompt_options())
 
     def _make_style_from_name_or_cls(self, name_or_cls):