Как в pyautogui нажать на среднюю кнопку мыши?

Читаем в документации про клик: https://pyautogui.readthedocs.io/en/latest/mouse.html#mouse-clicks

To specify a different mouse button to click, pass 'left', 'middle', or 'right' for the button keyword argument:

pyautogui.click(button='middle')

Чтобы просто нажать - читаем чуть ниже: https://pyautogui.readthedocs.io/en/latest/mouse.html#the-mousedown-and-mouseup-functions

If you want to perform these actions separately, call the mouseDown() and mouseUp() functions. They have the same x, y, and button. For example:

pyautogui.mouseDown(button='middle')