fix ListItem* repr for python 2

Fixes unicode errors when the item text is unicode.

Before:
````python
In [1]: from kivy.uix.listview import ListItemButton, ListItemLabel
In [2]: ustr = u'\u20ac'
In [3]: ListItemButton(text=ustr), ListItemLabel(text=ustr)
Out[3]: 
(<repr(<kivy.uix.listview.ListItemButton at 0x7f76b87c9a10>) failed: UnicodeEncodeError: 'ascii' codec can't encode character u'\u20ac' in position 21: ordinal not in range(128)>,
 <repr(<kivy.uix.listview.ListItemLabel at 0x7f76b876b738>) failed: UnicodeEncodeError: 'ascii' codec can't encode character u'\u20ac' in position 20: ordinal not in range(128)>)
````

After:
````python
In [1]: from kivy.uix.listview import ListItemButton, ListItemLabel
In [2]: ustr = u'\u20ac'
In [3]: ListItemButton(text=ustr), ListItemLabel(text=ustr)
Out[3]: (<ListItemButton text=€>, <ListItemLabel text=€>)
````