Fix SSH key loading for ed25519 and ECDSA keys

Unencrypted ed25519 and ECDSA keys crashed with an uncaught
SSHException, and the DSA fallback raised NameError since paramiko 4
removed DSSKey. Load the key with paramiko.PKey.from_path instead,
ask for the passphrase only when the key is encrypted, and log an
error instead of crashing when the key can't be loaded.

Require paramiko >= 3.2 for PKey.from_path.

pyproject.toml

@@ -33,7 +33,7 @@ classifiers = [
 [project.optional-dependencies]
 JOURNAL = ["systemd-python"]
 NOTIFICATIONS = ["dbus-python"]
-REMOTE = ["paramiko"]
+REMOTE = ["paramiko>=3.2"]
 
 [project.urls]
 Homepage = "https://git.shouldnt.work/fuxino/simple_backup"