Try various RFC conversions to ``datetime`` or return ``None``.

    :param date_str: Date string.
    :type  date_str: ``string``
    :return: Date time.
    :rtype:  :class:`datetime.datetime` or ``None``

def dt_from_header(date_str):
    """Try various RFC conversions to ``datetime`` or return ``None``.

    :param date_str: Date string.
    :type  date_str: ``string``
    :return: Date time.
    :rtype:  :class:`datetime.datetime` or ``None``
    """
    convert_fns = (
        dt_from_rfc8601,
        dt_from_rfc1123,
    )

    for convert_fn in convert_fns:
        try:
            return convert_fn(date_str)
        except ValueError:
            pass

    return None