Fixes macOS piping error by forcing urwid to use selectors.SelectSelector

fixes: #155
see: urwid/urwid#1025

piping with urlscan was broken on macOS due to issues with
selectors.KqueueSelector, which is the default on macOS.

This overrides the default on macOS to fix the problem.

Author: exprez135

urlscan/urlchoose.py

@@ -36,6 +36,8 @@
 if platform == 'darwin':
     COPY_COMMANDS = ('pbcopy',)
     COPY_COMMANDS_PRIMARY = ('pbcopy',)
+    import selectors
+    selectors.DefaultSelector = selectors.SelectSelector
 elif 'WAYLAND_DISPLAY' in os.environ:
     COPY_COMMANDS = ('wl-copy',)
     COPY_COMMANDS_PRIMARY = ('wl-copy --primary',)