sha256 checksum, thanks to: https://gist.github.com/rji/b38c7238128edf53a181

def _checksum(in_file, block_size=65536):
    """sha256 checksum, thanks to: https://gist.github.com/rji/b38c7238128edf53a181
    """
    cs = hashlib.sha256()
    with open(in_file, "rb") as f:
        for block in iter(lambda: f.read(block_size), b''):
            cs.update(block)
    return cs.hexdigest()