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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 ClientCertFromCSR(cls, csr):
 """Creates a new cert for the given common name. Args: csr: A CertificateSigningRequest. Returns: The signed cert. """

    builder = x509.CertificateBuilder()
    # Use the client CN for a cert serial_id. This will ensure we do
    # not have clashing cert id.
    common_name = csr.GetCN()
    serial = int(common_name.split(".")[1], 16)
    builder = builder.serial_number(serial)
    builder = builder.subject_name(
        x509.Name(
            [x509.NameAttribute(oid.NameOID.COMMON_NAME, str(common_name))]))

    now = rdfvalue.RDFDatetime.Now()
    now_plus_year = now + rdfvalue.Duration("52w")
    builder = builder.not_valid_after(now_plus_year.AsDatetime())
    now_minus_ten = now - rdfvalue.Duration("10s")
    builder = builder.not_valid_before(now_minus_ten.AsDatetime())
    # TODO(user): dependency loop with
    # grr/core/grr_response_core/config/client.py.
    # pylint: disable=protected-access
    ca_cert = config_lib._CONFIG["CA.certificate"]
    # pylint: enable=protected-access
    builder = builder.issuer_name(ca_cert.GetIssuer())
    builder = builder.public_key(csr.GetPublicKey().GetRawPublicKey())

    # TODO(user): dependency loop with
    # grr/core/grr_response_core/config/client.py.
    # pylint: disable=protected-access
    ca_key = config_lib._CONFIG["PrivateKeys.ca_key"]
    # pylint: enable=protected-access

    return RDFX509Cert(
        builder.sign(
            private_key=ca_key.GetRawPrivateKey(),
            algorithm=hashes.SHA256(),
            backend=openssl.backend))