Return a DER-encoded length field

    Based on code from python-ecdsa (https://github.com/warner/python-ecdsa)
    by Brian Warner.  Subject to the MIT license.

def btc_tx_der_encode_length(l):
    """
    Return a DER-encoded length field

    Based on code from python-ecdsa (https://github.com/warner/python-ecdsa)
    by Brian Warner.  Subject to the MIT license.
    """
    if l < 0:
        raise ValueError("length cannot be negative")

    if l < 0x80:
        return int2byte(l)
    s = ("%x" % l).encode()
    if len(s) % 2:
        s = b("0") + s
    s = binascii.unhexlify(s)
    llen = len(s)
    return int2byte(0x80 | llen) + s