bpo-25720: Fix the method for checking pad state of curses WINDOW (#4164)

bpo-25720: Fix the method for checking pad state of curses WINDOW (#4164)

Modify the code to use ncurses is_pad() instead of checking WINDOW
_flags field.  If your platform does not provide the is_pad(), the
existing way that checks the field will be enabled.

Note: This change does not drop support for platforms where do not
have both WINDOW _flags field and is_pad().