Make inputs and backup_dir optional in the config file

They can come from -i and -o; missing ones were rejected as a bad configuration file (exit code 6).

tests/test_config.py

@@ -224,9 +224,15 @@ backup_dir=/backups
         sb._read_config(config_file, USER)
 
 
+def test_inputs_and_backup_dir_optional(tmp_path: Path) -> None:
+    config_args = sb._read_config(write_config(tmp_path, '[backup]\nkeep=2\n'), USER)
+
+    assert config_args['inputs'] is None
+    assert config_args['output'] is None
+    assert config_args['keep'] == 2
+
+
 @pytest.mark.parametrize('text', [
-    '[backup]\nbackup_dir=/backups\n',
-    '[backup]\ninputs=/tmp\n',
     '[other]\ninputs=/tmp\n',
     'inputs=/tmp\n',
 ])