get_rsa_key_fingerprint(is_private=True) called load_pem_private_key(password=None),
so an encrypted private key raised a raw TypeError. add an optional password param
forwarded to the load; unencrypted keys ignore it.
verified: encrypted private key fingerprints with its password and matches the
public key's fingerprint; missing password still raises.
Signed-off-by: disqualifier <dev@disqualifier.me>