Improve requests.adapters.HttpAdapter.__init__ (#5266)

The `max_retries` argument can be `None`. It's handled here as `retries`:

https://github.com/urllib3/urllib3/blob/main/src/urllib3/util/retry.py#L235

stubs/requests/requests/adapters.pyi

@@ -53,7 +53,11 @@ class HTTPAdapter(BaseAdapter):
     config: Any
     proxy_manager: Any
     def __init__(
-        self, pool_connections: int = ..., pool_maxsize: int = ..., max_retries: Union[Retry, int] = ..., pool_block: bool = ...
+        self,
+        pool_connections: int = ...,
+        pool_maxsize: int = ...,
+        max_retries: Union[Retry, int, None] = ...,
+        pool_block: bool = ...,
     ) -> None: ...
     poolmanager: Any
     def init_poolmanager(self, connections, maxsize, block=..., **pool_kwargs): ...