fix: build failures with older cython 0.29 series (#1232)

src/zeroconf/_services/registry.py

@@ -25,6 +25,8 @@
 from .._exceptions import ServiceNameAlreadyRegistered
 from .info import ServiceInfo
 
+_str = str
+
 
 class ServiceRegistry:
     """A registry to keep track of services.
@@ -76,7 +78,7 @@ def async_get_infos_server(self, server: str) -> List[ServiceInfo]:
         """Return all ServiceInfo matching server."""
         return self._async_get_by_index(self.servers, server)
 
-    def _async_get_by_index(self, records: Dict[str, List], key: str) -> List[ServiceInfo]:
+    def _async_get_by_index(self, records: Dict[str, List], key: _str) -> List[ServiceInfo]:
         """Return all ServiceInfo matching the index."""
         record_list = records.get(key)
         if record_list is None: