mgr/orch: fix mypy errors

mgr/orch: fix mypy errors

Fixes:

```
orchestrator/__init__.py:6: note: In module imported here:
orchestrator/_interface.py: note: In member "yaml_representer" of class "DaemonDescription":
orchestrator/_interface.py:1039: error: Argument 1 to "represent_dict" of "SafeRepresenter" has incompatible type "ItemsView[Any, Any]"; expected "Mapping[Any, Any]"
orchestrator/_interface.py: note: In member "yaml_representer" of class "ServiceDescription":
orchestrator/_interface.py:1178: error: Argument 1 to "represent_dict" of "SafeRepresenter" has incompatible type "ItemsView[Any, Any]"; expected "Mapping[Any, Any]"
orchestrator/_interface.py: note: At top level:
orchestrator/_interface.py:1181: error: Argument 2 to "add_representer" has incompatible type "Callable[[SafeDumper, DaemonDescription], Any]"; expected "Callable[[SafeDumper, ServiceDescription], Node]"
Found 3 errors in 1 file (checked 29 source files)
```

Signed-off-by: Sebastian Wagner <sewagner@redhat.com>