tools/spec: add instance variable type hints for Registry

tools/spec.py

@@ -14,15 +14,14 @@ def rename(name: str):
 
 ###############################################################################
 class Registry:
+    types: Dict[str, object] = {}
+    extensions: Dict = {}
+    features: Dict = {}
+    applied: Set = set()
+
     def __init__(self):
-        self.types = {}
-        self.extensions = {}
-        self.features = {}
-
         self.types['API Constants'] = Unscoped('API Constants')
 
-        self.applied = set()
-
     def _serialise(self, name: str, queued: Set[str]):
         if name in queued:
             return []