gh-90102: Fix pyio _isatty_open_only() (#125089)

Spotted by @ngnpope.

`isatty` returns False to indicate the file is not a TTY. The C
implementation of _io does that (`Py_RETURN_FALSE`) but I got the
bool backwards in the _pyio implementaiton.

Lib/_pyio.py

@@ -1806,7 +1806,7 @@ def _isatty_open_only(self):
         """
         if (self._stat_atopen is not None
             and not stat.S_ISCHR(self._stat_atopen.st_mode)):
-            return True
+            return False
         return os.isatty(self._fd)
 
     @property