PyYaml’s Serializer: annotate open, close, serialize (#8383)

As seen here:
https://github.com/yaml/pyyaml/blob/master/lib/yaml/serializer.py#L27-L41

Neither `open` nor `close` methods return, so `None` added as the return type.

Finally, serialize accepts a Node and does not return as seen here:
https://github.com/yaml/pyyaml/blob/master/lib/yaml/serializer.py#L46-L58

stubs/PyYAML/yaml/serializer.pyi

@@ -1,6 +1,7 @@
 from typing import Any
 
 from yaml.error import YAMLError
+from yaml.nodes import Node
 
 class SerializerError(YAMLError): ...
 
@@ -16,9 +17,9 @@ class Serializer:
     last_anchor_id: Any
     closed: Any
     def __init__(self, encoding=..., explicit_start=..., explicit_end=..., version=..., tags=...) -> None: ...
-    def open(self): ...
-    def close(self): ...
-    def serialize(self, node): ...
+    def open(self) -> None: ...
+    def close(self) -> None: ...
+    def serialize(self, node: Node) -> None: ...
     def anchor_node(self, node): ...
     def generate_anchor(self, node): ...
     def serialize_node(self, node, parent, index): ...