Refactor tenacity/config.py to add type annotations

Updated the singleton implementation and added type annotations to various methods and attributes in the DictConfig class. This improves code readability and helps with static type checking.

tenacity/config.py

@@ -35,18 +35,18 @@ class DictConfig:
     - __repr__: Returns a string representation of the configuration object.
     """
 
-    _instance = None
+    _instance: t.Optional["DictConfig"] = None
     _lock = Lock()  # For thread safety
 
-    def __new__(cls):
+    def __new__(cls) -> "DictConfig":
         if cls._instance is None:
             with cls._lock:
                 if cls._instance is None:
                     cls._instance = super().__new__(cls)
         return cls._instance
 
-    def __init__(self):
-        self._config = {}
+    def __init__(self) -> None:
+        self._config: t.Dict[str, t.Any] = {}
 
     def set_config(self, **kwargs: t.Any) -> None:
         """Sets multiple configuration parameters."""
@@ -80,7 +80,7 @@ def get_config(
             config.update(override)
         return config
 
-    def reset_config(self):
+    def reset_config(self) -> None:
         self._config = {}
 
     def get(self, name: str) -> t.Any: