Fix: __unicode__ must return unicode

[dpetukhov]

Because if str(self.parent) returns something like \xd0\xbf\xd1\x80\xd0\xb8\xd0\xb2\xd0\xb5\xd1\x82 and you try to print(ModelClosure), you will get UnicodeEncodeError

I see this error in Django Admin interface on delete confirmation page when I try to delete User which has linked Comments which have CommentClosure instances saved in database. And django admin trying to show string representation of all objects and fails.

Reference:

• https://docs.python.org/2.7/library/stdtypes.html#string-formatting (note 6)
• https://docs.python.org/2/library/functions.html#unicode

[dpetukhov]

Probably I need to add some tests to actually prove the issue and fixes.