use a more natural method to check if self._text is unicode.

this is to handle the following case
```
class LabelText(unicode):
    def do_something():
        return repr(self)

Label(text=LabelText("some Message!"))
```
which would cause kivy to crash prior to this fix
Merge pull request #1998 from joranbeasley/core_text_fix_py2_gettext_for_unicode_inherited_classes

use a more natural method to check if self._text is unicode.