gh-139145: Fix tkinter event loop in interactive mode (GH-152257)

gh-139145: Fix tkinter event loop in interactive mode (GH-152257)

When a Tcl command running its own event loop (such as vwait or
wait_variable) was active and the user typed input on stdin, the event
loop kept spinning at 100% CPU.  The stdin file handler is now removed as
soon as input becomes available.

Also fix gh-139816: an exception raised in a callback no longer stops the
event loop to wait for Enter on a Python built without readline; pending
callbacks keep running until input is actually available on stdin.

Co-authored-by: mdehoon <mjldehoon@yahoo.com>
Co-authored-by: Christopher Chavez <chrischavez@gmx.us>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>