'Construct a Signer instance from a string.
Args:
key: string, private key in PEM format.
password: string, password for private key file. Unused for PEM
files.
Returns:
Signer instance.
Raises:
NotImplementedError if the key isn\'t in PEM format.'

@staticmethod
def from_string(key, password='notasecret'):

    parsed_pem_key = _helpers._parse_pem_key(_helpers._to_bytes(key))
    if parsed_pem_key:
        pkey = RSA.importKey(parsed_pem_key)
    else:
        raise NotImplementedError('No    key    in    PEM    format    was    detected.    This    implementation    can    only    use    the    PyCrypto    library    for    keys    in    PEM    format.')
    return PyCryptoSigner(pkey)
