Don't require 'exit' to be called with parentheses.

ptpython/repl.py

@@ -104,6 +104,11 @@ def run_and_show_expression(self, expression: str) -> None:
             except BaseException as e:
                 self._handle_exception(e)
             else:
+                if isinstance(result, exit):
+                    # When `exit` is evaluated without parentheses.
+                    # Automatically trigger the `ReplExit` exception.
+                    raise ReplExit
+
                 # Print.
                 if result is not None:
                     self._show_result(result)