yaml.dump(..., stream) is optional

<http://pyyaml.org/wiki/PyYAMLDocumentation#dumping-yaml>
> yaml.dump accepts the second *optional* argument, which must be an open
> text or binary file. In this case,  yaml.dump will write the produced
> YAML document into the file. Otherwise, yaml.dump returns the produced
> document.

Author: pmhahn

third_party/2and3/yaml/__init__.pyi

@@ -1,4 +1,4 @@
-from typing import Any, Iterator, Sequence, Union, IO
+from typing import Any, Iterator, Optional, Sequence, Union, IO
 from yaml.error import *  # noqa: F403
 from yaml.tokens import *  # noqa: F403
 from yaml.events import *  # noqa: F403
@@ -22,10 +22,10 @@ def safe_load_all(stream: Union[str, IO[str]]) -> Iterator[Any]: ...
 def emit(events, stream=..., Dumper=..., canonical=..., indent=..., width=..., allow_unicode=..., line_break=...): ...
 def serialize_all(nodes, stream=..., Dumper=..., canonical=..., indent=..., width=..., allow_unicode=..., line_break=..., encoding=..., explicit_start=..., explicit_end=..., version=..., tags=...): ...
 def serialize(node, stream=..., Dumper=..., **kwds): ...
-def dump_all(documents: Sequence[Any], stream: IO[str]=..., Dumper=..., default_style=..., default_flow_style=..., canonical=..., indent=..., width=..., allow_unicode=..., line_break=..., encoding=..., explicit_start=..., explicit_end=..., version=..., tags=...) -> Any: ...
-def dump(data: Any, stream: IO[str]=..., Dumper=..., **kwds) -> Any: ...
-def safe_dump_all(documents: Sequence[Any], stream: IO[str]=..., **kwds) -> Any: ...
-def safe_dump(data: Any, stream: IO[str]=..., **kwds) -> Any: ...
+def dump_all(documents: Sequence[Any], stream: Optional[IO[str]]=..., Dumper=..., default_style=..., default_flow_style=..., canonical=..., indent=..., width=..., allow_unicode=..., line_break=..., encoding=..., explicit_start=..., explicit_end=..., version=..., tags=...) -> Any: ...
+def dump(data: Any, stream: Optional[IO[str]]=..., Dumper=..., **kwds) -> Any: ...
+def safe_dump_all(documents: Sequence[Any], stream: Optional[IO[str]]=..., **kwds) -> Any: ...
+def safe_dump(data: Any, stream: Optional[IO[str]]=..., **kwds) -> Any: ...
 def add_implicit_resolver(tag, regexp, first=..., Loader=..., Dumper=...): ...
 def add_path_resolver(tag, path, kind=..., Loader=..., Dumper=...): ...
 def add_constructor(tag, constructor, Loader=...): ...