fix: restore normal screen on ctrl-z

[proskur1n]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

Right now, if you run any urwid example and try to suspend the application with ctrl-z (SIGTSTP), your terminal window will be left in a mess. This fix restores the normal screen buffer on ctrl-z, as urwid already does on exit. The following patch is based on the source code of ncurses.