Prevent flickering when redrawing the TUI python window

tui_win_info::refresh_window first redraws the background window, then
tui_wrefresh draws the python text on top of it, which flickers.

By using wnoutrefresh for the background window, the actual drawing on
the screen is only done once, without flickering.

gdb/ChangeLog:

2021-05-24  Hannes Domani  <ssbssa@yahoo.de>

	* python/py-tui.c (tui_py_window::refresh_window):
	Avoid flickering.