def _encode_auth(auth):
    """
    A function compatible with Python 2.3-3.3 that will encode
    auth from a URL suitable for an HTTP header.
    >>> _encode_auth('username%3Apassword')
    u'dXNlcm5hbWU6cGFzc3dvcmQ='
    """
    auth_s = urllib2.unquote(auth)
    # convert to bytes
    auth_bytes = auth_s.encode()
    # use the legacy interface for Python 2.3 support
    encoded_bytes = base64.encodestring(auth_bytes)
    # convert back to a string
    encoded = encoded_bytes.decode()
    # strip the trailing carriage return
    return encoded.rstrip()