Calculate property name from tag name, e.g. a:schemeClr -> schemeClr.

def _prop_name(self):
        """
        Calculate property name from tag name, e.g. a:schemeClr -> schemeClr.
        """
        if ':' in self._nsptagname:
            start = self._nsptagname.index(':') + 1
        else:
            start = 0
        return self._nsptagname[start:]