fix: annotate the unit path set so mypy accepts service_packages (NetOrk/netork#1236)

napalm_device_types/service_packages.py

@@ -82,7 +82,8 @@ def _report(output: str) -> Dict[str, List[str]]:
 def _units(lines: List[str]) -> Dict[str, Set[str]]:
     """``systemctl show`` blocks -> ``{unit: its unit file and binary}``."""
     units: Dict[str, Set[str]] = {}
-    unit, paths = "", set()
+    unit = ""
+    paths: Set[str] = set()
     for line in [*lines, ""]:
         key, _, value = line.partition("=")
         if not line.strip():