returns unicode sha1 hexdigest for sequence `seq`.

    >>> seq_sha1('')
    'da39a3ee5e6b4b0d3255bfef95601890afd80709'

    >>> seq_sha1('ACGT')
    '2108994e17f6cca9ff2352ada92b6511db076034'

    >>> seq_sha1('acgt')
    '2108994e17f6cca9ff2352ada92b6511db076034'

    >>> seq_sha1('acgt', normalize=False)
    '9482340281b5fc8f2a298dbbd6b82fe42159b6c5'

def seq_sha1(seq, normalize=True):
    """returns unicode sha1 hexdigest for sequence `seq`.

    >>> seq_sha1('')
    'da39a3ee5e6b4b0d3255bfef95601890afd80709'

    >>> seq_sha1('ACGT')
    '2108994e17f6cca9ff2352ada92b6511db076034'

    >>> seq_sha1('acgt')
    '2108994e17f6cca9ff2352ada92b6511db076034'

    >>> seq_sha1('acgt', normalize=False)
    '9482340281b5fc8f2a298dbbd6b82fe42159b6c5'

    """

    seq = normalize_sequence(seq) if normalize else seq
    bseq = seq.encode("ascii")
    return hashlib.sha1(bseq).hexdigest()