read a string element, maybe encoded in base64

def _parse_string_el(el):
    """read a string element, maybe encoded in base64"""
    value = str(el)
    el_type = el.attributes().get('xsi:type')
    if el_type and el_type.value == 'xsd:base64Binary':
        value = base64.b64decode(value)
        if not PY2:
            value = value.decode('utf-8', errors='replace')
    value = _uc(value)
    return value