<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_avatar(from_header, size=64, default="retro"):
 """Get the avatar URL from the email's From header. Args: from_header (str):
 The email's From header. May contain the sender's full name. Returns: str: The URL to that sender's avatar. """

    params = OrderedDict([("s", size), ("d", default)])
    query = parse.urlencode(params)
    address = email.utils.parseaddr(from_header)[1]
    value_hash = sha256(address.encode("utf-8")).hexdigest()
    return "https://seccdn.libravatar.org/avatar/{}?{}".format(value_hash, query)