Validates that 'value' is an instance of `basestring` for Python 2
    or `str` for Python 3.

def validate_string(option, value):
    """Validates that 'value' is an instance of `basestring` for Python 2
    or `str` for Python 3.
    """
    if isinstance(value, string_type):
        return value
    raise TypeError("Wrong type for %s, value must be "
                    "an instance of %s" % (option, string_type.__name__))