bpo-13886: Skip PTY non-ASCII tests if readline is loaded (GH-30631)

Skip test_builtin PTY tests on non-ASCII characters if the readline
module is loaded. The readline module changes input() behavior, but
test_builtin is not intented to test the readline module.

When the readline module is loaded, PyOS_Readline() uses the readline
implementation. In some cases, the Python readline callback
rlhandler() is called by readline with a string without non-ASCII
characters.