gh-140146: Use Tcl_CreateChannelHandler to enable piping to stdin with tkinter on Windows

[mdehoon]

When using tkinter in an interactive Python session, the EventHook function in Modules/_tkinter.c runs the Tcl/Tk event loop, and exits once data becomes available on stdin (i.e., when the user types in the next Python command).

The EventHook function monitors for input on stdin using Tcl_CreateFileHandler.

As Tcl_CreateFileHandler is Unix-only, for Windows instead the _kbhit function from Microsoft Windows is used.

However, this prevents stdin from being used with a pipe (see Issue #140146 ), as _kbhit checks for a keystroke on an actual keyboard.

This PR replaces Tcl_CreateFileHandler by Tcl_CreateChannelHandler and Tcl_GetStdChannel(TCL_STDIN), which do not depend on platform.

This has two advantages:

1. It allows stdin to be used as a pipe on all platforms, which is useful for testing interactive usage of tkinter;
2. We can get rid of some Windows-specific code in Modules/_tkinter.c.

• Issue: Tkinter on Windows cannot use pipe for stdin #140146