> I have an idea to solve it. But I don't know how to get the 
> clipboard data.

In Windows, using the window manager entails extending the process and the current thread with GUI-related structures in the kernel and then connecting the process to a window station (usually "WinSta0", which contains the clipboard) and connecting the thread to a desktop (usually "Default"). This permanently changes how the OS sees the process. I think whether or not the process should be a GUI process is something for the application to decide, not the standard library. Thus getpass should not read text from the clipboard.

The docs could note that terminals may need to be configured to support Ctrl+Shift+C (copy) and Ctrl+Shift+V (paste) shortcuts, and that some terminals provide alternate ways to paste text, such as a right-click action or context menu. I don't think the docs should provide detailed explanations and configuration details for particular terminals.