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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def retrieve(url):
 """Retrieve and parse PEM-encoded X.509 certificate chain. See `validate.request` for additional info. Args: url: str. SignatureCertChainUrl header value sent by request. Returns: list or bool: If url is valid, returns the certificate chain as a list of cryptography.hazmat.backends.openssl.x509._Certificate certificates where certs[0] is the first certificate in the file; if url is invalid, returns False. """

    try:
        pem_data = urlopen(url).read()
    except (ValueError, HTTPError):
        warnings.warn('Certificate URL is invalid.')
        return False

    if sys.version >= '3':
        try:
            pem_data = pem_data.decode()
        except(UnicodeDecodeError):
            warnings.warn('Certificate encoding is not utf-8.')
            return False

    return _parse_pem_data(pem_data)