avocado.utils.astring: ignore "undefined" unicode

Depending on the Python version stack being used *by inspekt*, it
can produce this error:

   Syntax check FAIL
   ************* Module avocado.utils.astring
   E0602:305,32: is_text: Undefined variable 'unicode'

Let's disable that check, since that variable is a reserved word
that should always exist in Python 2.7, for which a conditional
exists.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/utils/astring.py

@@ -302,7 +302,7 @@ def is_text(data):
     each character.
     """
     if sys.version_info[0] < 3:
-        return isinstance(data, unicode)
+        return isinstance(data, unicode)   # pylint: disable=E0602
     return isinstance(data, str)